Hi, I just want to share my experience and hopefully this will help someone out there with similar situation.
If you are getting something saying "the mail was undeliverable and it was returned to USCIS". In addition to call the 1-800 number they told you to open a service request and you should go to the local office as soon as you can. Luckily, I went on behalf of my parents right before the interview date. If I didn't go, we could miss the interview without even knowing it. The officer from the local office was able to pull the file and find the copy of our intervie notice.
Good luck
Good Day got a reaction from Nik+Heather in I-485 denied - never received interview letter