While testing ideas for a wither skeleton farm in a creative world, I cleared a perimeter with a glass floor covering the entire floor. Endermen are spawning on the glass floor. There are no other blocks available for them to spawn, so I know they aren't spawning elsewhere, and then teleporting to the glass blocks. They are spawning on the glass. I read that this had been reported in a snapshot, and marked as "Resolved", but I'm using the latest 1.14 release, and it is still a problem.