Not sure if is a bug or I'm missing something. Every time i'm creating a new shapefile with "Create shapefile layer" i got a multipolygon instead of a polygon (at least is what I'm expecting).
Also got same issues, when using an import of PostGIS table (with "Polygon" type geometry in db) and trying to add/save the polygons, I got an error with the message of unable to commit because different type inside the database compared to the default type which is created.
I ve tried different versions of QGIS (3.10,3.12,3.16) but seems to have all buggy behaviour. I'm on Windows.