With the release of 1.14 there was a change in the mob spawning algorithm, mainly involving the height map for how high up a chunk the game checks for mob spawning. In 1.13 transparent blocks did not affect this height map, this was only affect by solid blocks. Allowing mob farms to be build at bedrock so the spawning algorithm takes place faster. In 1.14 transparent blocks affect the height map even though they didn't before, meaning rates of farms at bedrock are severely reduced if you have glass at the surface. 
Here is a good video explaining the issue at hand, Link to a video by Methodzz

If this issue could be fixed, being able to have glass above my farms would be a lot nicer than having to have a large hole in the ground that wasn't previously necessary