Please try to reproduce the issue on the sandbox site - hidden link and make sure the problem exists or not. It will help us a lot in debugging and we can escalate the ticket directly to our compatibility team.
Thank you for your help Bigul, but it seemed really complicated because apparently ACF and WPML together have an incompatibility that was driving me crazy.
I found a solution with a bit of a hack because I can't seem to make this work the usual way :')