I ended up using the regular caption field.
It is no problem to show the attachment custom field under article thumbnails or gallery images, but if you put this field to the regular caption of images used to put in the tinyMCE there are some drawbacks:
– you have to do “disable_captions” (and then generate your own …) – that deletes the regular caption field from the editor 🙁
– the ACF attachment meta field is not available in the image editor, you have to go the library and edit the image information there to add content to the ACF field
This is confusing I think. Now i added the regular caption to the gallery and the post thumbnails.
