I don’t know of the solution in this specific case because the fact that you tried switching themes means that what I think might cause it does not make much sense.
This can happen if you have enabled local JSON files by creating the acf-json folder in your theme and ACF is unable to overwrite the existing JSON file for the field group. This causes ACF to use the old version in the acf-json folder instead of the new version from the database on posts, but ACF will show the updated version in the editor.
If you have acf-json folders in your themes, delete the folders from the server. If this is the cause of your problem then it should be solved. If it is then you need to find out what the write permissions of the folder and/or existing files in the folder. It seems that if they are created by ACF in the first place that ACF should be able to overwrite them.
If this isn’t the problem, then any information you can provide about your effort to solve it might help others.
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