Client upgraded to the premium version of the plugin and it works okay –?I think it’s an issue with the free version of the plugin only, so something missing in the code perhaps to interpret the URL correctly.
The github issue has been updated, and the fix in in the beta of picturefill.
I’ve updated the plugin locally for now, but it’d be good to know if/when the change will be ported (I’m guessing when it goes public instead of beta)?