No hostile mobs spawning naturally anymore

On my server, no hostile mobs (Zombies, Skeletons, Creepers, etc.) are spawning naturally any longer.

I am not running in peaceful mode, doMobSpawning is enabled and there are no command blocks that kill any monsters. I can spawn them using spawn eggs, but none are spawning naturally at night, in dark areas / underground or even around mob spawners I found in an abandoned mineshaft.

I tried setting the difficulty to peaceful and then back to hard again, and I tried disabling the doMobSpawning rule and re-enabling it. The only hostile mobs that do spawn every now and then are Phantoms, and mostly in groups of 3 or 4.

All worked fine up until a few days ago. Sadly, I cannot remember when exactly this problem started, but it may have with the 18w48b update.

There are no log entries concerning mob spawning whatsoever.