Plants are not uprooted when building a tile if there's a drywall

I want to uproot these two weezworts by replacing their dirt patch with a wall, but none of them where uprooted in the process.

I think it's because the dirt patches were created by heating algae, and there was a drywall behind. So, the weezewort were planted by pips on a "double" tile : dirt + drywall.

And when my duplicant build the insulated tile, he doesn't dig the dirt, but replaced the drywall.

This seems confirmed by the fact that I don't get dirt piles after building the tiles.