Patrols can spawn in the ground, causing them to suffocate. I have experienced this issue twice in my survival world. The first time this happened, the patrol spawned right in front of me, on the other side of my local river, and some of the pillagers were inside the hill, suffocated, and died. The second time was today, when I saw an ominous banner just dropped on the ground. There were no iron golems, or really any mobs nearby, so I can only assume they spawned in the ground, suffocated, dropped the banner, and then it popped out on the surface.