It signs out depending on that tenants policy. In my case I got 4 tenants connected with one requiring a relogin every now and than. The layout is also odd, slack and discord do it better but you can have a convo in one tenant and a call in the other at the same time.

Shared channels between organizations are still a pain as they have to be configured by IT - both orgs need to trust each other. Makes sense but it's a pain for users.