Are you using or do you have gutenberg enabled? If so is it up to date?
If not can you please let us know what theme if any and what editor if any.
There is a well known bug that seems to show it’s face if you are using gutenberg. It was meant to have been fixed (please make sure your wordpress and gutenberg are up to date) however there are recent reports this could still be an issue:
Original report and confirmed bug here: https://github.com/WordPress/gutenberg/issues/11280
Confirmed bug fixed here: https://github.com/WordPress/gutenberg/pull/11409