Sounds like your file is damaged. Launch FileMaker without opening this file and use Recover from the File menu to open and produce a recovered copy of the database. If this is a hosted database, you'll need to do this on a local copy of the database file.
Even if the recover process reports "no problems found", test the recovered copy and see if it still has this issue.
Things to keep in mind about Recover:
While Recover almost always detects and fully corrects any problems with your file...
- The recovered copy may behave differently even if recover reports "no problems found".
- Recover does not detect all problems
- Recover doesn't always fix all problems correctly
- Best Practice is to never put a recovered copy back into regular use or development. Instead, replace the damaged file with an undamaged back up copy if this is at all possible. You may have to save a clone of the back up copy and import all data from your recovered copy to get a working copy with the most up to date information possible.
And here's a knowledgebase article that you may find useful: What to do when your file is corrupt (KB5421).