Version: 1.7.2-BETA+1.17
Modpack: All of Fabric 4

With a mob spawner that is three blocks tall, with a cursed earth floor, and the floor covered with express conveyor belts, endermen are not pushed by the conveyor belts.

also happens on 1.17.1

If you're talking about this:

Then it's actually intended, as the conveyor block has its own collision box. Just build it a block taller.