Issue seems to persist in 20w19a. Spawn rates for mobs have increased slightly, however it is still not the same as in 20w16a. Tested in both an enderman farm, and a general mob farm. World updated to 20w19a created in 20w17a show no problems. Only affects worlds created before 20w17a.
Seems to only affects worlds that were updated to 20w17a. Making a new world and building an enderman farm yields expected rates.
Fixes the issue, thank you.