Acacia trees under y=0 often grow with bare branches

While playing on a multiplayer server I noticed that acacia trees with 2 branches grown under y=0 sometimes formed with one empty branch without a leaf canopy.

Steps to reproduce:

1. Place an acacia sapling under y=0.
2. Grow the sapling using bonemeal or by waiting for random tick updates.
3. Repeat steps 1-2 until an acacia tree with a split trunk is produced.
4.  Notice that the tree has one branch that is bare.

Expected outcome (grown above y=0):

4.  Notice that both branches on the tree have a separate canopy attached to them.

 

Note that this does not apply above y=256. All attached screenshots are taken in 21w13a.