Some people do not know what completion means. Some people have chat turned off or don’t speak English. I actually ask everyone in the group if they are cool with staying past the timer. If they say “we can time it” I say “cool but if we don’t are you staying or not” then if I don’t get a yes from someone I kick them out and replace them before putting in the stone.
When people aren’t reliable, tyrannical leadership and super clear communication works wonders.