Yeah, tea washing usually works, as does a shorter infusion. 150 degrees is pretty cool for an black tea, so no doubt you reduced the extraction efficiency.
Try washing, using the 3 minutes at a slightly warmer temp, maybe 180 degrees, and report back.
Also would help to know what the flavoring is - maybe it extracts more quickly than the tea liquor itself, so that it imparts a bitter flavor (like the relatively nonvolatile fatty ester chocolate flavoring).
Hadn't thought about the fannings angle that Victoria mentions, but its got to have an impact on bitterness, because it remains in the extract after the tea filter is removed. Good point. Helps explain why very finely leaved teas tend to yield a bitter cup at under typical brewing conditions (3-4 minutes) at normal infusion strength - look for the dregs at the bottom of your teacup.