PyQt5 QGraphicsItem odd inheritance behaviour

This is a problem I've found the solution to, but not the answer to. The following is the relevant part of my code:

class MyClass(QGraphicsPolygonItem, MyAbstractGraphicsShapeItem, MyGraphicsItem)
    def __init__(self):
        super(MyClass, self).__init__()

The first class is the type of graphics object I'm trying to create. For me, the first one I did was a polygon. The second class is one which needed to inherit QAbstractGraphicsShapeItem to change the objects color through. The third class works with any QGraphicsItem. To make everything modular and reusable, this was my setup.

The issue I faced was as such:

1. If the first class was after the second class, MyClass would no longer recognize it had a polygon I could set.
2. If the third class was before the second class, MyClass would stop painting itself, showing up in no way on screen.

As you can see, the sequence shown in my code is the only one that works. One of the sequences (third, first, second) even crashes the program. Here is what I think happens:

The graphic item classes inherit in this order, from superclass <--- subclass.
QGraphicsItem <--- QAbstractGraphicsShapeItem <--- QGraphicsPolygonItem
When the initialization priority is the same as the inheritance direction from subclasses to superclasses, the inheritance works.

The Point

This is what I think the initialization order looks like in these situations:

1. MyAbstractgraphicsShapeItem, QGraphicsPolygonItem, MyGraphicsItem, QAbstractGraphicsShapeItem, QGraphicsItem
2. QGraphicsPolygonItem, MyGraphicsItem, MyAbstractgraphicsShapeItem, QAbstractGraphicsShapeItem, QGraphicsItem
3. QGraphicsPolygonItem, MyAbstractgraphicsShapeItem, MyGraphicsItem, QAbstractGraphicsShapeItem, QGraphicsItem

Where the third is the working one.
I cannot see the difference between them. I know how it should work but not why. The initialization order shouldn't matter, as the super call should order them like this, where there are no conflicts between functionality. I know that QAbstractGraphicsShapeItem doesn't have boundingRect or paint functions, but I believe they should be overwritten by QGraphicsPolygonItem class functions. I would like an answer, because this was a really baffling issue to solve.

Edit:
I'll add the other two classes, just the relevant parts:

class MyGraphicsItem(QGraphicsItem):  
    def __init__(self):  
        super(MyQGraphicsItem, self).__init__()  

class MyAbstractGraphicsShapeItem(QAbstractGraphicsShapeItem):
    def __init__(self):
        super(MyAbstractGraphicsShapeItem, self).__init__()

For additional detail, MyGraphicsItem allows me to place graphics items in relation to the parent as I wish, while MyAbstractGraphicsShapeItem allows me to create button like behavior (since QAbstractButton only works with QWidgets).