field content existing for a field would not cause this. The fact of a field being required or not is stored in the field definition and not in the field data for a post.
The only thing that I can possible think of that might affect this is local acf-json files in ACF5. If you have created a folder in your theme for local json then try deleting the json file for the effected field group.
If that’s not it then try standard debugging steps, deactivating all other plugins and switching one of the default WP 20XX themes.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
Helping others is a great way to earn karma, gain badges and help ACF development!