This is sounding way too familiar for me. So here are my two cents.
I-751 is stuck somewhere between the lock box you sent it to and the field office for whatever reason. (too many applications, hiding in a box somewhere, file fell of a desk. You get the picture). You then file the N-400. This is like hitting the emergency brake. The NBC demands your file to be sent to them immediately and everybody starts looking for it. ones it's found it gets sent to the them and you get put into the cue for the N-400.
If the N-400 application is ready and complete your file gets send to your field office. The field office then puts you in line for the N-400 interview and starts working on the I-751 as well. if both applications are correct and ready around the same time, it's combo interview time. If one is done way before the other you get whatever interview is ready first and have to wait for the other.
N-400 cannot get a final approval before the I-751 is approved and closed out.
I had my I-751 jarred loose by filing the N-400. Had that interview way before the N-400. Than had the N-400 interview while the I-751 was still waiting on the final decision. Received a letter with the "recommending approval" of the N-400 while leaving the office after the N-400 interview. I received the approval notice for the I-751 shortly after that and then the approval notice for the N-400.
When you filed your service request, did you do that for the 751 or the 400? The I-751 may be outside the window (no idea what the use for it nowadays) but is irrelevant as your N-400 trumps it.
The clock for the N-400 window started the moment they received your N-400 application. I checked the site and it does. say June is outside, but 09/18/2024 is the first date that returns "within normal times" so that 9.5 is optimistic.
You need to find out where your actual file is, I was able to have them look it up in their system for me during my info pass appointment for a 551 stamp. Your local congress person may be able to get some info for you.