In my mob farm made with cursed earth, the endermen just stand there unaffected.

Just tested it and endermans are indeed being moved by conveyors belt. You were probably not giving them enough room to be pushed.

Endermen are 3 tall, make sure you give them enough room to walk as if the room is 3 tall and the conveyer belt is 1/10 tall that puts the enderman inside of the ceiling (you could line the roof with halfblocks if you want to narrow the space as much as possible)