Not sure if it’s an issue as such, although I struggle to think of a scenario where you would want this functionality, and it might be obvious to some but just as an FYI
If you send multipolygons into a Dissolver with Aggregate Handling set to Deaggregate and choose attributes to sum, the polygons will be deaggregated first, this means that each constituent part inherits the attributes of the original multipolygon feature and this will then be summed to leading to unexpected values.
So from this
You get this
