[Закрытые] Flexible content fields not translating

Background of the issue:
Our client has reported multiple occasions where their flexible content fields are gone after copying over content to another language. We have fixed the issue multiple times by setting all ACF fields to 'Copy once' in the 'Custom Fields Translation' WPML setting. The site also has ACF extended installed and also has acf-json Autosync enabled. At the moment the issue is fixed but our client wants to prevent it from happening again in the future. It is hard to replicate the issue if we do not know what is causing it. Let me know if you need any extra access to a staging environment to debug further.

Symptoms:
Flexible content fields are gone after copying over content to another language.

Questions:
How can we prevent flexible content fields from disappearing after copying content to another language?
Is this issue related to WPML or ACF?
Do you need access to a staging environment to debug further?