My guess is the "booster" of only 5gr might not be enough to gain any fps. Some T7 is likely sticking in the barrel and mixing with the BH209 too much. The idea behind a duplex is to raise pressures enough that the main charge reaches its required pressure curve quicker. T7 does spike pressures faster and BH209 has a broader/longer curve without being higher than straight T7. The noticeably lower fps is still a bit odd though.

Maybe a 10gr booster and 75gr BH209 would provide some insight. Trying the same experiment with a 209 primer breach plug might also or maybe with a 250gr bullet to see if it continues to produce less fps.