Natural mob spawn rates plummet in mid to late game after exploration

I have a realm that I regularly play on with friends, and recently mob spawn rates in the overworld and the end have plummeted to almost nothing. Notably, the change was seen after doing a whole lot of exploring using elytra in the end and the overworld. It seems that mobs in unloaded chunks are being included in the mobcap. Mob spawners still spawn normally, and drowned spawn rates don't seem to have changed.