I closed the room and set the infinitive light so that it only shines through the windows.

Can't really see any leakage.

To be frank, hardly anything commercially available is truly PP-2012 or PP-2014 compatible.

Most Poser users still rather clinge to older versions and that's the clientel the majority of merchants still cater for.

I think the only solution will be to add primitives in Poser to stop the leakage or make the seams overlap in a modelling app.