This is a very rare bug that has no real reproduction steps except to exit and enter the end repeatedly. I basically just did that and eventually I got this weird glitch where all of the endermen spawned right by (technically right above) the obsidian platform. I did not leave the main end island, so it is not a result of spawn constricting (where in the end you go 144 blocks away and come back and all the endermen are on one side of the island). The seed is 2144793778238684365.

My end portal was manmade right at spawn, in case that is related somehow.

My suspicion is that this can occur in other dimensions as well, although I have not seen such strong evidence.

My guess for the cause of this is that it is related to all of the chunks-sometimes-not-loading-properly issues in recent snapshots, and for some reason these were the only chunks considered as valid to spawn mobs. Could also be a partially implemented fix for MC-215444.

The (forced crash) crash report attached was done quite a while after this issue was experienced, but my player had not moved. (I had the game running while I was submitting this report.)

My render distance was set to 17 while experiencing this bug.

Possibly caused / caused by MC-225063, as this was experienced in the same world.

In 21w19a this occurred but all the endermen where in a straight 1-chunk line for some reason