Comment 33 for bug 1830747

As Yang noticed, uuid is missing in scheduler_hints as well as in instance_groups. After our upgrade from Mitaka (previous upgraded from Liberty) to Rocky we had same problem. Not only cold migration/resize was failing, also "nova-manage db online_data_migrations --max-count 50" was failing when instances with instance_groups are found.
As workaround we solved it by inserting scheduler_hints directly to DB (use on your own risk):

--UPDATE nova_api.request_specs rs
INNER JOIN nova.instances ins ON ins.uuid = rs.instance_uuid
INNER JOIN nova_api.instance_group_member igm ON rs.instance_uuid = igm.instance_uuid
INNER JOIN nova_api.instance_groups ig ON ig.id = igm.group_id
SET rs.spec = REPLACE(rs.spec, ', "scheduler_hints": {}},', CONCAT(', "scheduler_hints": {"group": ["', ig.uuid, '"]}},'))
WHERE ins.deleted = 0 AND rs.spec LIKE '%"policies"%' AND rs.spec NOT LIKE '%"uuid":%' AND ins.uuid = '3cdd744e-243e-4328-8323-970a9e038c0e'