This bug was present in 1.14 as well, but players in an McMMO party who go to the skyblock world mess up the party permissions. They are somewhat "kicked" out of the party but it seems like they aren't really. When they return to survival, they are unable to chat in the party, and have to rejoin it. Rejoining causes all party messages to be displayed twice.
These symptoms lead me to think this is a permissions issue. My educated guess is that players are not given enough permissions for McMMO party chat in skyblock, and these permissions are not restored in survival. Ideally, player's should have permission to chat in party when in skyblock but not gain party xp. If that isn't possible, players should have their party permissions restored when returning to survival