Sloped surface destroyed by stacker+rock

Issue description

I have a bulk storage, mainly for looks, where I dump dirt and rocks for later use with stackers.
(dedicated miners and trucks manage dumping when there is a dumping area)
It just looks so good.
I have noticed that dumping rock destroys sloped surfaces, just the sloped ones the flat ones are staying.
Dirt does not do that

My guess is that depositing rock tricks the surface on slopes that it is not smooth anymore and surface is destroyed as if the ground were mined out under it.

Reproduction steps

1. Load save
2. Go to bulk storage
3. Build surface on the missing slopes
4. Fill attached storage with rocks and let stackers drop rocks
5. After the excavators mined the rock up again, the surface is destroyed

Game version and build number

v0.6.3b b334

Operating system

Windows

Attachments

chicken_.save

Update:
If the dirt gets grassy, then dirt removes surface too

My guess is that grass is somehow the culprit, since the bottom of rock has a trasition layer of dirt which can be grassy

Update2
It might be a save loading issue.
Some grassy dirt did not destroyed the surface