There's been an issue with a red steles that give -30% speed and -30% defense in dark rooms that when in co-op, all joined players are affected by it in all rooms like they are dark rooms, but disabling it stops the effect on them. I tried to look at the wiki to see where it would be located since they are unique to areas but the wiki does not have that info. I have not ran into this bug with any of the others so I don't know if this is a recurring issue with any others