Hi,
Thank you very much, I was able to find the problem.
There is another field group under the ACF options, which was in the trash. When requesting the field options from ACF, it returned the one that was “trashed”, and so no valid options were returned. Once I restored the group from the trash, it started working again.
It has the same field names, so that is probably the issue. You can leave that group as it is, or perhaps delete it permanently, to aviod the field name duplicates.