Missing Blocks And Enderman Spawns In End

I have started a project in my end. Im making a large black concrete box with the outside full of water so no endermen spawn, and inside will be decorated with a village or something. The first problem i have is that my massive completed black concrete box, L-115 B-122 H-120, sometimes when reloading back in after world close or a refresh chunk load, small parts of the walls are destroyed for some reason. My second problem is the Enderman. I went around the outside of by box, LITERALLY everywhere and placed water so that enderman cannot spawn anywhere whatsoever outside, meaning they can only spawn inside my box. It sort of worked, they dont spawn outside because of the water, like I wanted, but they dont spawn inside my box, which is what i want. Inside my box there isnt any slabs, water or any deep holes, so where actually are they... I’ve did all the restarts of my xbox, Minecraft, swithing difficulty... still the same... they do spawn tho hundreds of blocks away from the main island in the end city... I’m really confused.