I have had this problem repeatedly in different flavours over the past year, every time caused by the “blocks” feature, and some third party conflict.
In all cases, either disabling the Gutenberg editor on those posts, or removing the third party, resolved the issue.
So that is where I would suggest to start:
1. Try without the Gutenberg editor
2. Try disabling plugins, or switching themes (you may want to do this on a staging site, or at least after performing a backup of your site)