Illager patrols spawning on village roofs

I keep having patrol parties spawn inside my village.  I watched a party spawn in on the roof of a house.  The wiki says they're to spawn on grass or sand.

Because they keep spawning inside my village, they keep wiping out my villagers before I even know they're there - if I'm mining or in my house, etc.

Attached is a screenshot I got after the party dropped off the roof - was too surprised to snap quicker.

EDIT:  Got a 2nd screenshot of a party that had spawned on the roof of my horse barn.  There's no way they could have climbed up there.  This is just outside the town boundary.