Maybe she leaves to Coruscant after the battle. Maybe she’s requested for an emergency session of the Imperial Senate where some good old Emperor decides that due to the Empire being attacked it is about time to dissolve the Congress. Maybe.
The Emperor dissolved the Senate before the attack on the Death Star.
Well, this film does take place before the attack on the Death Star. So there are plenty of potentially good explanations for why Mon Mothma would be on Yavin IV for a scene in RO, but not during the events of ANH.