I’ve been having a small issue while modifying and saving field groups.
When having images fields with return values set as “image array” I would get image IDs as output. To solve this, I would then have to open the field group, change the selector from “image array” to “image ID” and back again to “image array” and save the changes.
It has happened a few times, so before solving it the way I did I tried generating the export code and found out that the field had an empty string as return format. Like this: 'return_format' => '',