All level 50 flat battles set a Pokemon's level to 50, if the level is below 50 it would either set it to fifty I think or leave the Pokemon at that level.
Now I know that wasn't the question, but y'know, just in case or whatever.
As for rejections of over 50s occurs in generation three or at least emerald by experience and likely also in gen 2 in all battle formats I think, purely by memory.
I am however aware that generation four, gen 5, 6 and 7 all set a Pokemon's level to 50 and don't reject them,if they are lower then 50 then they will stay at the level they are at in battle, because we love our focusend rattatas.
Haven't done this in a long time,