First Channel After Login cannot be a group/Private channel

After upgrading reluctantly from my perfectly stable 3.18.7 to the latest 6.5.0 I noticed that on login I was getting an error:

Room not found
The room does not exist or you may not have permission.

It would appear that First Channel After Login can no longer be a private channel. e.g. companytalk.

This is a private server with no public login but users login first to this private channel.

But private channels now have a URL containing 'group' eg

https://chat.somedomain.com/group/companytalk

Open channel contains 'channel' eg

https://chat.somedomain.com/channel/generalstuff

If we leave First Channel After Login set to companytalk we get the above error. If we change the channel to open/public the /group/ changes to /channel/ and we login correctly as expected.

Also on logout in a browser it should return to:

https://chat.somedomain.com

But it always ends up at this which is the First Channel.

https://chat.somedomain.com/channel/companytalk

Docker
Rocket 6.5.0 and earlier versions

Reproduce the login and browser logout flows with a private First Channel After Login, using the Rocket.Chat 6.5.0 Docker setup described in the issue. Trace the route handling for the private /group/ URL and the logout redirect; done means private first channels load after login and logout returns to the server root rather than the channel URL.