AJAX product attribute selection resets other attribute values to default

I have an issue with product attributes which I sure may have been reported before - if so then apologies but I can't find it....

I have more than one set of attributes applied to a product, in my case "Size" and "Style".

When I view the product display node and change the first attribute option, the second attribute option resets to the default option.

Step 1
Load node with default choices: -

Size
[ ] Medium
[*] Large
[ ] X Large
[ ] XX Large

Style
[ ] Print Only
[ ] Framed
[*] Block Framed

Step 2
Change second option: -

Size
[ ] Medium
[*] Large
[ ] X Large
[ ] XX Large

Style
[*] Print Only
[ ] Framed
[ ] Block Framed

Step 3
Change first option: -

Size
[*] Medium
[ ] Large
[ ] X Large
[ ] XX Large

Style
[ ] Print Only
[ ] Framed
[*] Block Framed Here's the problem - it's gone back to Block Framed again

If someone knows which AJAX / PHP function call is responsible for rendering the product add-to-cart form I can probably add a patch, but I can't find it right now.

Thanks,

David