The problem is that the stockable plugin is not using the standard vm method to load a product. So the normal stuff of deriving values from the parent do not work as expected.
We have two different methods to display stockable variants. One method is by openglobal, great js stuff and nice to handle. But it is old vm1 style, also the reason vm1 veterans prefer this method.
The other method "dynamical child variants" is using the vm methods to load a product. All parent/child stuff is there working as expected.
The future is a bit different. I want to add for vm2.1 a stock for every customfield, so we dont need anylonger this complex constructions, just for the stock.
The real fun with this stuff is atm anyway most time not used. For example to change the product image or dimensions.