Shulkers can't spawn in survival mode

I tried to go to an end city, to kill some shulkers. But there was no shukers. Not one ! I exported the world, and try to go there in creative mode. And surprise, there were Shulkers !
I sought an other end city, and again : no shukers in survival mode, but in creative mode there were.
If the endcity is discovered in creative mode: shulkers do spawn
If the endcity is discovered in suvival mode: shulkers do not spawn.
If the endcity is discovered in survival mode, and then, explored in creative mode: shulkers do not spawn
If the world is in creative mode but you explore in survival mode, shulkers spawn.