It's not a bug, but rather a design oversight.
Currently, if a server has GM loaded, none GM owners even with the compatibility pack. can join the server, as it treats it like a CDLC server and X's the option to join out. Even if you load the datapack manually, you still cannot join from the launcher.
This means that players who have the datapack, will have to first manually load their mods, then join the server in-game. I feel this defeats the purpose of the launcher, as it serves as a great tool to help guide people into loading the correct mods.