Has the retention for the replicated volume had chance to expire for the snapshot(s) for which you had originally set 50% as the limit?
As you set this to 50% when you first created the Volume Collection, these settings would have been replicated to the downstream volume and wouldn't automatically change when you changed the VolCol (65% limit) on the upstream volume, this limit would change for all replicas from this moment forward but not for things that had already been replicated, they would only change when the last snapshot with those original properties had expired.
So if the original setting have not yet expired with retention, you will still get warnings of a 50% limit until they do expire.
If you believe they have expired, then I'd call support to investigate further.
I hope this makes sense?