After gathering a plant in caverns the tile underneath is "xxx cavern floor", whereas building a farm plot on the same cave grass and then deconstructing it creates clay soil tile. Same happens when you cut down a tree.
#Cave soil disappearing after plant gathering
ruby portal
ruby portal
Curiously, those cavern tiles can be set and used as clay collection zones, so maybe it's just a visual bug?
upper ice
They are in fact not soil. This is a sort of oddity with how floor tiles are determined; when you gather a plant, the floor's material is set to that of the layer material, which at cavern level is always going to be a stone. It's annoying, and should be fixed I think, but not technically a bug.
(this is an explanation for context, not saying you shouldn't have made the report--you should have and this should stay up)