My gf and I have been playing on a multiplayer server (she just hosts, not a dedicated), we've had no issues up until the last couple nights, where I'll lose connection to the host and get kicked to the main menu, however I can't rejoin, when I try joining through steam, it'll say she's no longer playing a game but has project zomboid open. She has to close and re-start the server for me to be able to join through steam, then I get about 10 minutes before it does it again. Has anyone had this kind of issue? I couldn't find one that matched this exact issue, and we've had no issue the last couple weeks prior.

Yep. This sounds quite similar to the issues that I've experienced with my son. It sucks I know, but they will get things squared away I'm sure. The build was released in a half baked state. Not a terrible state mind you as it does work for the most part, but it does still need some work before it's anywhere near as stable as build 40 was.