Gd.games version didn't fix joining but fix it now and I can't buy it
Its working fine on GD.games as well
My guess is: You can't join because the host lobby is auto closing when the host window is not focused, here is how you can join without any issues:
1- Open 2 separate tabs and open the game on them.
2- Enter the host and set host private lobby, while the other player hit join.
3- When the other player is in the (enter the host ID), make the host exit and create a new lobby, then enter the host new ID and hit Join.
You don't have to worry about this issue btw for players because each player window is focused anyway, but for testing you need to do that steps.
For more testing, you can also avoid all this steps and simply enter the game form 2 different devices and join the host without any issues.