She was in ROTS played by the same actress wearing different clothes.
The prequel trilogy is stupid.
Again, I suppose there’s some “logical” explanation why Mon Mothma does not appear at the Death Star attack run briefing, does not appear as the attack on the Death Star is unfolding, does not appear at the medal ceremony, does not appear anywhere on Hoth…
But now, hey! She’s hanging out in a leadership role in the Rebel alliance in a movie scene that predates the 1977 movie.
Disney did not waste any money on continuity experts.
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.
But that’s just what I would have done to solve the continuity issue. Perhaps they just do not care.