Here's what happens: I open the doc, the search function works as it should in the Navigation Pane, then suddenly it doesn't. The Navigation Pane is still open and maybe it'll show how many instances of whatever I'm searching for there are, but it doesn't show the actual text. Then it does other odd things, like showing "No" (instead of "None") if there are no hits to the search term. If I exit the doc and every other doc I have open, then go back into the doc, it'll work again, but then stop again, sometimes after only one search. This has happened on several docs. It's not about a corrupted file.
Version of Word: Microsoft Office 365 (the subscription service)
Product + update: Microsoft® Word for Microsoft 365 MSO (Version 2505 Build 16.0.18827.20102) 64-bit
Windows + update: I'm running Windows11 / my system is totally up to date (my system says so)
Safe mode: I have not tried safe mode and since I've changed utterly nothing in Word in more than a year, I seriously doubt anything would change if I did run in safe mode.
Screenshot: I cannot get this problem to replicate in a short document--I tried out test docs--and the documents it's occurring in are copyrighted, so I cannot show them here. Perhaps the doc has to be 80K or 90K words long (200 or 300 pages) in order for this problem to happen?
I'm horribly frustrated by this and it's making my job 100x more difficult than it already is.
And, btw, it's extremely difficult to get anything to post on this forum. If I get "Oops! Something went wrong. Please try again." one more time . . .
This is try #5
Please try starting Word in safe mode.
This is a diagnostic procedure. It is not hard to do.
It is an important one whether or not you have changed anything on your system.
While in safe mode, open one of the problem documents and run your search.
Do you have the same problem?
Open Office apps in safe mode on a Windows PC - Microsoft Office
Windows Version and Update
Try typing SysInfo in your Windows search box. I get the following:

Microsoft 365
- Personal?
- Family?
- Enterprise?
Education?
Screenshot
I am a lawyer. I understand confidentiality. I understand copyright.
You do not need to disclose anything confidential to post screenshot.
Make a copy of one of the problem documents. In that copy, insert some random text that is not confidential that will be on the screen.
Run your search with that random text still on the screen. Take a screenshot of the navigation pane in the Results tab.
Attach it in your reply.
How to include a screenshot in your post here in the Microsoft Community
It would help if I can see what you are seeing. Please try a screenshot showing each of the problematic results you mention.
As to dealing with this forum, I understand your frustration. I get the same oops message from. time to time. I just got it again trying to respond here! When this happens, I make a copy of what I am trying to post and refresh the browser page. Then post into a new reply.
I have not experienced the problem you report and do not really care to create a 300-page document to experiment. If nothing else works, I will though.
The people responding here are your fellow users. I do not work for Microsoft.
I also encourage you to post feedback about the problem. That gets seen by people who can make changes to the program rather than your fellow users. What we can do in this forum is help you use the program as it currently exists and perhaps help you find glitches in your settings. We can't make the program work better in general.
This is a user-to-user support forum. -- You can give feedback TO MICROSOFT designers and decision makers on Microsoft Word or other Microsoft Products using their feedback mechanisms, (not here). (This is a link to directions.)