Providing more information about your database and computer system would be helpful to those reading your post. There are design elements that can fool the user into seeing what you are seeing and without knowing more about your system, we can't rule that out.
On the other hand, issues with the file's indexes can prevent finds and other operations from functioning as expected. You could try:
1) recover a copy of the file and see if you see the same issue in the recovered copy.
2) save a clone (no records) and import all your data into the clone. Test this copy to see if your problem persists.
If that fixes your problem, read up on file recovery and decide which of the above files to keep for future use. It's been a "best practices" advice for many years not to use a recovered copy of the file unless you have no alternative.