This issue is weird. When i click "Play Solo" game for some reasons starts a client and a server, when its only supposed to open client side. Whats wrong? This issue is not in latest stable release! In latest stable release, it opens properly. While in beta it opens 2 separate windows....

Probably because Play Solo works differently on both versions. In the beta, it starts a server and connects to it. I'm not sure why, maybe it was made to let players use chat in solo as in the release build, you can't chat in solo

This isn't a bug at all. Novetus Snapshot starts a Client and Server for solo as it was the original way Roblox did it (Roblox would start a server in the backend for one player, and then it opens a client to join it). This makes testing places and features for multiplayer much easier, as places in solo would function differently to how they'd work online. This means chat, leaderboards, and everything would work the same as it would on a normal Novetus/Roblox server.