Just an update for anyone else who gets this problem:
This only occurs when a default variation has been chosen ( in this case size ).
To fix it, don’t set a default size/colour/etc!
I’ll update this thread but I’ve opened an issue if anyone else wants to follow along :
Sorry for the delay. Sorry I hadn’t tried this before, yes the same thing happens on Twenty Twelve and so is a woocommerce issue. Any idea how to fix this? Has anyone mentioned this before?