Dark oak tree roots can break bedrock

Steps to reproduce

(See Dark oak.mp4.)

1. Make a bedrock platform.
2. Place 4 dirt blocks on the bedrock in a 2x2 pattern
3. Place 4 dark oak saplings on the dirt.
4. Place a column of 7 dirt above the north-west sapling.
5. Bonemeal the north-west sapling.

Expected result

The tree grows upward or doesn't grow.

Actual result

Sometimes the tree grows downward and branches replace bedrock.

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Placing dirt blocks over the top of a dark oak tree causes it to sometimes form roots rather than a tree. If there's enough free space and no other logs, the tree will grow in some way. These roots can replace bedrock, then be broken by a player in survival.