Since a PC update to 1.0.1.501, this seems to have broken cross platform play between Mac/Linux versions and PC versions of the game.
Aspyr Media - the developers of the Mac/Linux version of Civ 6 - said on twitter on 29th May that:
Maya & Gran Colombia Pack is available now
This suggests an update to be inline with the Windows version, yet the bug still persists.
When playing on the "Internet" option, Mac/Linux users are presented with the error: "Version Mismatch", while playing on "Unified PC Play", an unspecified error occurs when connecting
Does anyone know if there is a workaround to force Civilization to not check the version since they are running the same build (to my knowledge)?