I'm unable to get foxes to harvest sweet berries that have been planted on mud.

What I expected to happen was...:
I expected them to harvest sweet berries as they do on dirt.

What actually happened was...:
The foxes harvested once when placed in the farm, but did not harvest again.

Steps to Reproduce:
1. Build a sweet berry farm, using mud as the base block for planting.
2. Add foxes.
3. See the lack of sweet berries being harvested.

From the attached screenshots, I built an identical farm with dirt (aside from hopper minecarts imbedded in the dirt blocks) and the foxes seem to have no issue harvesting sweet berries.