How to reproduce

1. Create a superflat world in creative that has the preset: 

   minecraft:bedrock,5*minecraft:stone,5*minecraft:dirt,5*minecraft:sand,minecraft:water;minecraft:deep_ocean;oceanmonument,biome_1
   

2. Get away from any shipwrecks and ocean ruins (so that the enderman can't teleport there)
3. Spawn an enderman in the water and it wont teleport anywhere
4. Run the command:

   /fill ~ ~ ~ ~63 ~ ~63 minecraft:cobblestone_slab[waterlogged=true]
   

5. Spawn an enderman on said giant slab of submerged cobblestone slabs
   → Notice that the enderman is teleporting

Note

The reason why this is a glitch is because of the inconsistency of endermen teleporting or not in the water.