Hi everyone,
This issue has already been report here:
https://support.advancedcustomfields.com/forums/topic/hidden-required-field-prevents-post-to-be-update/
A ticket has been sent to the support.
Hopefully it will be fixed very soon.
Meanwhile as @gaialabs said, I don’t think this topic should be consider as Solved.
I can’t remove the “Solved” solution, apparently. Anyway I don’t think we should debate this here.
Also, I got an answer from the support team and they will fix it in version 5.7.9.