I triggered a Bad omen pillager raid on purpose, and 2 mob kills later, the boss bar displayed "Raid: 2 Mobs Remaining". Later observations showed that exactly 4 mobs spawn every time, none of which spawn witches or illager beasts, both of which normally spawn near the end of pillager raids triggered in this manner.

This bug probably occurs either due to the mass slime spawns in superflat worlds, or due to the fact that I was using the default superflat world, which had its topmost layer at y=4.