When spawning pillagers, solid blocks such as wood logs covered by leaf are used as a spawn spot for spawning pillagers. On the screenshot, we can see while most of the pillagers inside a patrol spawn on the ground, one of them is stuck on the top of a tree and is inside a leaf block, just above a solid block (a wood log), which suggests the pillager just spawned there. This situation of pillagers growing in trees is frequently seen while playing.

Analysis: one of the conditions for pillagers to naturally spawn is on a solid block which has a sky access. Yet, leaf blocks do not block sky access. The spawning process seems to forget to check whether there is enough space for the pillager to spawn, especially on the lowest block.