This has been happening for many years, probably since Multiplayer was added, and it's still unsolved.

Technically not a bug I guess, but it can get annoying.

When adding a Server IP on the Multiplayer menu, if by accident there's a space at the beginning/end of the IP, that'll count as part of the IP address and will make it impossible to join that server. It's not a huge issue, but sometimes it can be hard for Server administrators to recommend this if a player is having trouble joining their server because of how simple the issue is.

The solution is simple. Just automatically remove/ignore spaces on the IP address.