We group creatives together using their Creative ID. The Creative ID is assigned by Meta Ads Manager when the asset is uploaded.
That means "Group by Creative" combines any creatives with the same Creative ID.
If you're seeing the exact same creative show up more than once in a report grouped by creative, it's probably because the asset was uploaded to Ads Manager more than once (rather than duplicated).
To avoid this, duplicate in Ads Manager, rather than uploading an asset multiple times. Then Motion will cleanly bundle together all the matching creatives.
For ads with different creatives for different placements:
These will also bundle together.
Only true dynamic creatives will be broken up when you use "Group by Creative".