Some decorations in the tileset have an opaque background
Causes visual bugs like this, where the sand below the cactus cuts into the dune:
The specific example is in https://gitlab.com/TMW2/clientdata/-/blob/master/graphics/tilesets/desert1.png with more occurances, like the cow skull, some more cacti and others.
It is very likely that this is also affecting other tilesets, so please check and possibly clean up some of the tilesets!
The fix can be as simple as using gimp to transform the sand into alpha and modifying the map.