After collecting 8 creepers in my creeper farm and nametagging them, I killed 4 with a bow and 4 more would not spawn. From this point on no more than 4 creepers (nametagged or not) will spawn at any given time. The mob cap for this area seems to have been reduced for some reason.
4 Skeleton traps at the bottom of my farm (approx. 180 blocks below) that had spawned during a thunderstorm were causing this issue. Killing them restored the 8 count of creepers.