I have an ACF Pro subscription, and when I contacted support about a website I run, they asked me to post this here. I apologize if this is not the right place to post this.
Currently, the problem in my environment has been resolved (by resetting the DB), but I don't have the time to execute the following again to reproduce the problem. Please forgive me.
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  1. I put the ACF field group in the "Trash". (This was a mistake.)
  2. Next, I restored it from the "Trash".
  3. Then, "__trashed" was automatically added to the end of the field key of some fields (I wonder why it was some and not all).
For example, what was originally
field_1212121212121
became...
field_1212121212121__trashed
  1. This caused me a lot of trouble because I could not load the field correctly from themes or programs.
  2. I have now restored from a DB backup and this problem on "my website" has been resolved.
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However, I think this problem is too fatal to be caused by a one-click mistake (putting it in the trash).
The issue of "__trashed" being automatically added to the slug of a "post" that has been put in the trash seems to be something that is not limited to ACF, but I have not been able to identify the cause of this problem.
*You can find several example cases by searching "WordPress __trashed" on Google.
What I hope for the time being is that this problem would occur less often if it were no longer possible to accidentally put something in the "trash" from the ACF custom field list screen.
For example, an alert could be displayed saying "Are you sure you want to put this in the trash?"
Of course, it would be best if __trashed was not added to custom field keys.
Thank you in advance for your consideration.