Cant play drop-ins in a lobby with "fill teammates disabled" setting

my friends tried playing 3-man drop-ins but 2 of us were rm builds and 1 was st build. i assumed we will be playing rcm and lcm like we would in a "fill teammates" enabled drop-in match but no it said "too many players in the same position. then i switched to rw and he stayed rm but still nothing. this needs to be looked at because if we were playing with "fill teammates" enabled it would just put us rcm and lcm