Just upgraded from 5.5.14 to 5.6 and I was unable to remove a field after updating it.
When editing a field group and adding a new field, I would click “Update.” After the page updates and I attempt to remove the field, it will fade away but when I click “Update,” it’s still saved in the field groups. Drag and Drop ordering saves successfully, but the fields won’t be removed.
I just tested this with several types of fields and I’m not having an issue deleting fields. Is it all fields or one field in particular? Can you supply more information of the field(s) that you cannot delete?
I am working on an nginx environment (not sure if it’s relevant, but just more detail). I resolved the issue by reinstalling ACF and now all fields function as normal.
To be more clear, all the fields I was trying to update or remove were created in v.5.5.14 and it was individual fields within a group that would not delete. Also, the field table layout seemed to break after the first time I saved by adding the “Key” column and forced “Type” to return onto a second row of table headers – these new columns couldn’t be removed with WP’s Screen Options.