Using New Outlook on Windows for personal email, calendar, and contact management
SOLVED IT! I had the same problem and tried all the settings here with no success.
Came across this post about Search Folders today and it worked! All my old emails showed up in the new search folder!
Short answer: set up a Search folder with a time filter like "older than three months."
https://www.nakivo.com/blog/how-to-find-old-emails-in-microsoft-365/
I don't understand why they aren't in the Inbox anymore but at least they aren't gone. Hope this helps others.