[2021] compatibility issue

A19.4 is the current stable version. You don't need to opt into it, Nor can you. The beta list only shows Old versions of the game, and the latest experimental where applicable. The current stable version is always accessed by opting out of all betas.

Whichever one of you is on A19.3 needs to go to the beta menu and select "None - Opt out of all betas" to update to A19.4.