Short answer
HubSpot's KB: accounts created after April 1, 2024 can't create tickets from inbox channels and use help desk. One ticket per conversation, and it can't be dissociated. Changing the conversation owner updates the ticket owner, not the reverse.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Plan a move to help desk if you're still on the legacy setup.