After synchronize fields with JSON files, the updated fields Translation preferences turns to “copy” mode instead keeping the original option “copy once”.
How to repeat the issue:
On localhost update some fields, all with Translation preferences = “copy once”.
Upload folder /acf-json/ to production server
On production server Sync fields.
Result: fields that had Translation preferences = “copy once” became “copy” mistakenly.
Advanced Custom Fields PRO Version 5.12.3
Advanced Custom Fields Multilingual 1.4.0
WPML Multilingual CMS 4.5.4