Moved blog – galleries don’t work

Gallery can include either:

• specific images, in which case their IDs are explicitly listed in shortcode
• all images attached to the post, in which case shortcode doesn’t specify that (as in your example)

So your description is a little contradictory – if images weren’t attached to the post, then that example wouldn’t have worked in first place.

Otherwise something in migration or update process might have disrupted IDs or parent/child properties, look into that.