Well, there isn’t really anything I can do, I’m not the developer.
Please submit a new support ticket here to get this to the developers https://support.advancedcustomfields.com/new-ticket/
Not really sure if they can do anything since ACF uses PHP’s json_enocode() an json_decode(). The reason that I asked what version of php you’re using is that I’ve seen other related problems because there are some incompatibilities with encoding and decoding in different php versions.
https://support.advancedcustomfields.com/new-ticket/
