Pig&Monkey

The USCIS approved your case and handed it off to the Dept. of State. It is the Dept. of State that conducts the SAO-M background checks, NOT USCIS. The NVC is Dept. of State, so they may or may not have additional information.for you.
 
The case most likely WAS sent back to the US as part of the SAO-M investigation. That is normal procedure. It is, as you say, "not actually returned". It goes to Washington, D.C., but NOT the USCIS.
 
Again - "Apart from the Department of State, the process involves other agencies of the federal government of the United States, especially those under Department of Homeland Security. Some of the agencies involved are: FBI, CIA, Drug Enforcement Administration, U.S. Department of Commerce, Department of Treasury's Office of Foreign Assets Control, and the Department of State's Bureau of International Security and Nonproliferation. The process may also include checking with Interpol."
 
You are unlikely to find out any additional information until the Consulate tells you the investigation is complete, but something may shake out when you rattle the cage.
 
Hang in there - that is all you can do until they get back with you.

A lawyer could only tell you what triggered the investigation. Maybe not.

It's good you're calling USCIS hotline - however, when you call in again, 
ask for a Tier 2 person, 
as they can see more on the computer system.  A Tier 1 cannot see much. 
 
They (a Tier 2) should be able to verify the location of the casefile, and current review status, and SHOULD BE able to see which codified reference # [pertaining to whatever law is on the books] that he's being reviewed for. 
 
Sometimes, having that info (casefile location and codified reference #) is useful.   

First of all, I am very sorry that your visa plans have been severely delayed. However, with respect to the state of affairs/relations between the two countries, trade relations/sanctions, swaggering and posturing by both leaders at each other, extradition of Chinese citizens back to the US, it is easy to see where the common folk become the grist for the grinding wheels of both countries. I was kind of wondering when and if either of the two governments would go down this type of restrictive path. Hopefully, your situation will be resolved in your favor soon. Hang in there.
 
Good luck on your immigration journey. 

You are within a reasonable time frame for that kind of background check. Refiling under a different category would simply restart the same clock.
To determine the reason would require a knowledge of your fiance's career, resume, past interactions with the consulate, and possibly those of family members.
To find out more, including possibly information about others in the same boat and your best strategy going forward, you should contact Marc Ellis at http://www.marcellislaw.com/. He has had extensive involvement with the GUZ consulate in cases like this.
Hang in there a little bit longer.

from Wikipedia
 
Security Advisory Opinion (SAO) or Washington Special Clearance, commonly called security clearance, administrative clearance, or administrative processing, is a process the United States Department of State and the diplomatic missions of the United States use in deciding to grant or deny a United States visa to certain visa applicants. The process involves sending a request from the visa issuing post to the Department of State's headquarters in Washington, D.C., to investigate an individual's case for possible espionage, terrorism, and illegal export of technology out of the United States.
 
 
 
Apart from the Department of State, the process involves other agencies of the federal government of the United States, especially those under Department of Homeland Security. Some of the agencies involved are: FBI, CIA, Drug Enforcement Administration, U.S. Department of Commerce, Department of Treasury's Office of Foreign Assets Control, and the Department of State's Bureau of International Security and Nonproliferation. The process may also include checking with Interpol.

Thanks for the info Randy! I know you've been a big help on here and Candle for Love. We understand this processing can take a very long time. Though I have seen others in TAL fields who've been approved in 1.5-2 months after submitting their resumes, we understand every case is different. But since we received that conflicting info from NVC about a possible case return to USCIS, we are very worried this means a denial. Would someone even notify us by now if we were denied? We have thought about withdrawing our K-1 and filing a CR-1 instead, but if we don't know why we were denied, we wouldn't be able to overcome it even if we file again. All I can think of now is that I'm going to try calling USCIS to see if I can find out anymore on Mon. We just feel very defeated.

Hi all,
 
I've been a long term reader of this very helpful and supportive site! As I've been reading so long, I'd like to share and ask if anyone is going through a similar situation as my fiance and me.
 
1) My fiance had his interview in Guangzhou on 2019-01-24. The interview was ~10 min, and the CO did ask about his PhD in Materials Science & Eng and also kept the resume my fiance brought. None of the instructions asked to bring a resume, but we wanted to be very prepared as my fiance is in a TAL field. Atfer the interview, the CO gave my fiance the blue slip with only "Administrative Processing" marked and informed him there were no problems and the processing would take ~3 wks. 

2) On 2019-01-28, we received a follow-up instructions email from the consulate with a Word doc attached named "SAO-M" (we presume SAO indicates Security Advisory Opinion-Mantis"??). When we opened the doc, the header was "Administrative Processing". The doc asked for my fiance's detailed resume in both English and Chinese (format resembled the DS-5535). And on 2019-01-29, we emailed the resume as instructed by the consulate, and we have been waiting every since..
 
3) We have emailed the consulate multiple times. I have called and emailed both my senators and my congressman. My congressman has been very helpful in contacting the consulate and then the DOS (Bureau of Consular Affairs). But each time, we still receive the same AP comment. I did call NVC last Fri, and received conflicting responses: 1st response said a note had been placed into the system stating the case has been returned to the USCIS on 2019-05-31. When I called again to confirm, the 2nd response said there was no such note. One of my senator's office did say the consulate will often return cases for further background checks.
 
Right now, we are just very confused and frustrated. Any tips would be much appreciated; thanks!