There seems to be a major bug regarding the built in function in the editor to access the unit group “Units Selected” (by a specific player). While I have no idea why it happens it seems that if you ever try to get the number of units selected by a player in a trigger for example with a condition that says:
Number of units in (Units currently selected by Player 1 (Red))) Equal to 0
then it will consistently cause issues in online multiplayer both with selecting units/structures or using abilities on the ones you currently have selected. I have yet to be able to confirm whether the same issue can also occur from using the unit group in other contexts than “Number of units in…”