This is an issue we've been experiencing in various RP instances.
Regardless of if a given instance is at or under soft cap - or below hard cap - random players will attempt to join an instance and then be sent off to a separate instance.
If we create a new instance, the issue no longer impacts those being afflicted with the issue above, but instead seemingly afflicts a random user attempting to enter the new instance.
When the issue happens, those effected see this in their output logs:
2021.09.22 22:36:03 Log - [Behaviour] Successfully joined room
2021.09.22 22:36:03 Log - [Behaviour] Spent 0.08389282s entering world.
2021.09.22 22:36:03 Log - [Behaviour] Waiting to enter network room
2021.09.22 22:36:04 Log - [Behaviour] OnConnected
2021.09.22 22:36:04 Log - [Behaviour] OnJoinRoomFailed
2021.09.22 22:36:04 Log - [Behaviour] Resetting game flow because "Failed to join room: ": [SkipLogin, TryAlternateInstance]
Again, this seems to happen regardless of a given instance's population size.