We recently upgraded our dedicated server to 1.14.20 and noticed our creeper farm which was working great prior to the update had suddenly stopped spawning in mobs.

The farm was made 200 blocks in the air, with no other spawnable blocks around it.. I have switched the difficulty from peaceful back to hard multiple times in an attempt to clear the mob cap. I was the only person in the server at the time of testing also.

It seemed like the glass platform that was used for the item collection platform (bottom layer of the farm) was blocking spawns on all the platforms above it.  Unfortunately I don't have a screenshot of the original glass platform, you can see part of it in the first screenshot.

The glass platform was more or less the same size of the spawning platforms above it.. as soon as I replaced the glass with stone the farm begun to operate normally again.

Not sure if this issue is the same with other transparent blocks.. but definitely is a problem with glass.

EDIT: As a sanity check, I removed all the stone and replaced it with glass once again.

To my surprise the mobs continued to spawn at the normal rate?! It makes absolutely no sense... A part of me wants to close the bug report, but something was definitely blocking spawns temporarily.. We have been trying to figure out what the cause of this problem since last night and the only thing that seemed to trigger any sort of change was removing the glass.