Private Servers currently do not support universes. This means if a developer of a large-game wants to support Private Sevrvers servers, they need to implement an entire system to support it themselves. I did this for Swordburst 2 around 5 years ago and it was not trivial (because this same feature did not support large-games then either, when it was called VIP servers)
It would be great if Private Servers were supported such that any TeleportService call made within a Private Server would only teleport to other instances created by this private server.
I considered filing this as a bug report, but I found a thread from 6 years ago showing this seems to be the desired behavior. IMO - this feature should work out of the box for large games. These devs have enough to deal with as-is, and rewriting Private Servers will never be a high priority!