I digitized somewhat complex (many vertices) boundaries of neighborhoods in QGIS as a Polygon shapefile. Turns out QGIS thinks they are Multipolygons and Tableau doesn't like them. They are not multipolygons as per the definition of a multipolygon.
I copied the features into a Polygon scratch layer and confirmed that the wkbType is Polygon. I then saved the scratch layer using both the Export - Save Features as and the Make Permanent options. In both cases, it converted the Polygon scratch data to a Multipolygon shapefile.
I have many uses of the data, mostly in QGIS and Python code which don't care, but Tableau is treating the polygons all as one polygon.