Mushrooms in swamps replace trees

See attached screenshot. Weirdly, it didn't only replace the bottom log of the tree, but also the one above that.

To reproduce (steps are this way because of MC-55596):

1. Create a new world with seed 7913388302499728605
2. /tp @p 1017 104 10157
3. Fly (don't teleport!) to 1017 65 9657
4. Look at the tree.