Due to incorrect usage of the tags attack_knockback (which doesn't seem to exist in 1.12, only in 1.14), and knockback_resistance (which should be knockbackResistance, whenever mobs with these attributes exist (or are spawned) in custom dungeons, the console spams this:

[11:23:31] [Server thread/WARN] [net.minecraft.entity.SharedMonsterAttributes]: Ignoring unknown attribute 'generic.attack_knockback'
[11:23:31] [Server thread/WARN] [net.minecraft.entity.SharedMonsterAttributes]: Ignoring unknown attribute 'generic.knockback_resistance'

Each custom spawner in each custom dungeon has to be looked at and changed accordingly - which might take a while.

Fixed every single Spawner in every single structure. Yay!