How to see the issue:
Open a drawing document. Go to Tools – Options – OOo Draw – Grid and set “Grid
range†to 30 pixels, so that you can better see snapping.
Then turn on “Snap to Object Pointsâ€.

1. example
Draw a traditional rectangle, slant it with -30° and rotate it with 20°. So it
is no more a rectangle and its edges are not parallel to the page edges.
Draw a line from somewhere outside to the figure. Move the mouse a little bit
around till the end point of the line will snap to the parallelogram. You will
see that it snaps to the corner of the parallelogram.

Now draw a parallelogram of the custom shapes and rotate it a bit. You can use a
shape from the list “Basic Shapes†or “Flowchartsâ€, it's the same.
Again, draw a line to the figure. Move the mouse around till the end point of
the line will snap to the parallelogram. You will see, that it doesn't snap to
the corner of the parallelogram but will snap to the edges of the invisible
surrounding box, which you see, if the parallelogram is marked.

2. example
Draw a traditional circle. Draw a line to the circle till it snaps. You see that
it snaps to the center of the circle, which is the only “object point†which
makes sense for a circle.

Now draw a circle of the type “custom shapeâ€, which you will find in the list
“Basic Shapesâ€. Again draw a line till the endpoint of the line will snap. You
will see, that it doesn't snap to any point of the circle, but will snap to the
surrounding box.

It seems to me, that a custom shape is handled as if it is a rectangle (=the
invisible box, which you see if you click on the object) and the figure is only
“painted†on that rectangle.

Expected behavior: “Object point†is a point a the figure.