This is not a bug where something is not working as intended, but a bug where things working as intended causes (presumably) unintended consquences that break other aspects of the game.  I play on a realm with 2-6 players online together at any given time.  Since the 1.18 update we have noticed problems with lag and with spawners we believe to be due to mobs spawning in unknown/unexplored caves. 

The main concern is spawners.  If a player AFK's near a spawner for any significant length of time, spawns at the spawner will slow rapidly and stop within about an hour.  Spawns only restart at the spawner if all players leave the area and return, which would despawn any hidden mobs.  Our assumption is that mobs spawning in unexplored caves are filling up the spawn limit and thus preventing spawners from functioning normally. 

We also get significant lag in and around the group base, but in different spots over time.  This does not coincide with where we have builds that might contribute to lag.  Again, having all players leave and return to the area (thus despawning any unseen mobs) clears up the lag... For a time.