[MC-125040] 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.

Linked issues

Greg Milson 2018-06-04T08:58:56Z

Is this still an issue in 18w22c?

Cannot confirm in 1.13-pre1.
There is neither tree nor mushrooms at that coordinates.

[Mod] redstonehelper 2018-06-04T22:01:45Z

The issue is most likely still present in the code, just the example we found doesn't exist anymore.

Without a working seed and coordinates, it is impossible to say whether this can still occur.

If anyone comes across that again, please provide a new seed + coordinates.