It is possible to have filters and actions in the theme that effect the admin. Also any function. functions.php and any files that this file loads are loaded in the admin. But if you’ve tried other themes then this should be eliminated.
I would say it could be a possible read issue if you are using local json, but since this requires having a folder in the theme folder, this should also be eliminated by changing themes.
I honestly have not idea what could be causing your problem.
At this point, if it were me, I would wait until it happens and then I would attempt to figure out why it’s not happening. I would do this by opening the main ACF file /plugins/advanced-custom-fields-pro/acf.php, and I would follow the code and find where the field group is and using echo, print_r and var_dump on variables in ACF I would see if I can figure out what’s going on.
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