Endermen do not spawn in the end.
You can make them spawn again by placing a block.
At this point the endermen can spawn at this one by one square. in every y coördinate as long as there is a block for them to spawn on, you can also remove the block you place to trigger the spawing of the endermen at this position. Because the endermen will not stop spawing wen you do remove this block.

This problem occured when I upgraded from 18w16a to 1.13-pre1, this problem is not fixed in pre2.