The cube relic is the last relic I have to unlock in the lobby, and I've found it many times in my runs (sometimes multiple times in a single run), but it never appears in the lobby.

There are several messages that you have to get before you get the Cube, so it takes a long time for you to actually find it.

I'll look into the code to double check, but I haven't heard anyone else bring this up.

You're right! The Cube has been compromised in the code. I'll put out a patch sometime soon to fix it.