I did further testing, it seems that naturally spawned witches become part of the raid if they are close enough when it starts, and this can cause issues when a witch is in a cave below the village that the player is unable to access or does not know about, as the raid will seem to be stuck with 1-2 mobs remaining due to the witches in the caves, maybe this behavior either should not happen, or there should be some sort of check if a witch is visible to the sky before making them part of the raid?
I did some looking into it, I had a similar issue (2 times) before, it seems it was due to raid mobs spawning in caves down below the area under the village, found them by going into spectator, and I do wish to note that both of the 2 times it happened, it was a witch that had spawned in the cave, but I am not sure if that is what will always happen, or if any of the raid mobs can have this issue.
Edit: I should add that both of two instances happened in 19w08, the first one in a and the second one in b
Having Vanishing entities issue with 1.14.2 (Running Paper as the server, but confirmed to also happen on vanilla when tested later), horse vanishes on dismount and only reappears on login