I think the issue is with it being just a single wall. The slicer tends to have some trouble with that.

My first thought was just to model the return paths as well, making it a loop, print that in vase mode, then just cut away the unwanted parts. My second thought was to wonder whether there would be a way to use post-processing to set the extrusion to zero for those particular paths, maybe using the multicolor features, and thereby avoid printing the extra area.