Glow lichens can spread outside of the world border

The Bug:

Glow lichens can spread outside of the world border.

Steps to Reproduce:

1. Locate the edge of the world border and place a glow lichen along the edge of it.
2. Apply several amounts of bone meal to the glow lichen.
3. Take note as to whether or not glow lichens can spread outside of the world border.

Observed Behavior:

Glow lichens can spread outside of the world border.

Expected Behavior:

Glow lichens would not be able to spread outside of the world border.