Endermans doesnt spawn at the end

We have killed the dragon, endermans was there, but i dont know why, one day when we were going to do the enderman farm, we saw that there wasnt any enderman on the island, was empty at all, 0 enderman, full empty.

i dont know why this happen if u can help me it will be nice, we have erase the end and create it again and i happened again, i think its because we go like 40000 x 40000 y to search end city, and that might bug that spawn