I am having the same issue but with the more recent snapshots. Wither spawns fine in 1.15 (and in 1.15 snapshots) but will not spawn in any of the latest snapshots on any difficulty or orientation.
Please fix this. You fixed the issue with the walls, trees, and observers. This seems like something that would be along the same line. This is a major issue that is keeping me from being able to finish my tree farm. 😞
Bobby GBs
I am having the same issue but with the more recent snapshots. Wither spawns fine in 1.15 (and in 1.15 snapshots) but will not spawn in any of the latest snapshots on any difficulty or orientation.