The resource packs work off realms if the world is down loaded so i don't think the resource packs are the issue conflicting with user accounts.
Does this happen for multiple realms or only a certain realm?
Only have/pay for the one realm, cannot confirm if has affected more than one.
Does this happen when a specific player logs on, or for any player?
Any player.
At what point in the connection sequence does the realm freeze?
Freeze occurs on loading screen just after the initialization of the building terrain sequence.
What happens after the realm freezes (messages, etc)?
Nothing its a straight program freeze. Has to be hard closed.
Crashing and failing to load resources packs on the Nintendo switch for 1.19.1 “failed to join realm” messages. But will get in after several resets. If lucky