Endermen spawn seems to be very rare in warped forests. I have built a platform at y = 250 for them to appear but only get something like three every ten minutes. 
I have read on the wiki that since 20w18, endermen were preventing other endermen from spawning nearby, but is it as drastic as only allowing them to spawn in packs of 1 once every few minutes ? 

I am doing this while playing alone on a server.