If a mob that is used in raid waves is present in a village once a raid begins, they count towards the raid's progress. Some people decorate their bases with mobs, and they would have to kill them in order for the waves to progress. I would imagine a fix would be to have raids only register mobs with a special nbt tag towards waves. Shouldn't be too tricky of a fix.
MC-147495
Mobs in village prior to raid count towards a raid once one starts
Resolved
Works As Intended
3
Unconfirmed
Minecraft 19w14a, Minecraft 1.14 Pre-Release 5, Minecraft 1.14
Retrieved
Affects 1.14 Pre-Release 2
This is game-breaking when there's an outpost not too far away, as the raid becomes infinite. Even when more pillagers spawn in after eliminating all in the wave, the wave reverts.