polymer toner is staying longer hot. Maybe rubbing hard on the B-Paper with a cloth will help you.
For me it makes no sense why the whole process is perfect except of these little dots.
This have nothing to do with out media. Maybe the toner did not fix there perfectly because you have to print two times with the HP Ghost printers?!
Maybe there are some uneven areas in your heat press or missing heat on the heating plate.
Please place the A and B sheets in another corner of the heat press to see if there is a problem with the heat press.