Hello Matthias,

Acymailing custom field is only for updating subscribe state. Means, it will only update the subscribe state for users that already in subscribers list ( http://take.ms/zSHCh ). Currently, the way Acymailing custom field works is by checking if the user is already in the subscriber list then only update the subscribe state based on user input.

This is not a problem for newly registered user since Acymailing will auto-add those user in that subscribers list. With that said, that field shouldn't be displayed when editing for user who is not yet in the subscribers list. I have added this fix in our repository and will be included in the next release.