Assign MATID processing algorithm does not support MultiPolygon geometries
The "Assign MATID" processing algorithm in the QGIS plugin creates bad output for source layers with multiple polygon geometries per feature.
This is due to the utility iterating over all features, getting the target MATID, and then collecting its geometries into a single polygon-like type, which is then used to identify mesh elements that should be reassigned.
Related: an option should be provided to select how this containment check is performed, i.e. full containment of the polygon, containment of all vertices, or containment of its centre of mass.