I tried to use the auto stitching function and found out it could not stitch areas that have other polygons carrying different signals.
So I deleted all the auto stitching vias and redid them manually, but after I did that, Altium left white outlines for the restrained areas I defined for stitching vias. I can't select those outlines or delete them.
See white outlines the orange arrows point at:
I tried highlighting the stitching via union but there is nothing: