A few things to check on During the smoke. which may lead to a bitter taste...
1) condensation dripping down from the top,,, I kinda doubt this considering the vent was wide open
2) The catch pan / tray for the pucks became dry .. letting the pucks burn causing the ambers,, and the burnt wood drift onto the meat hence the bitter taste.. keep a watch on this issue,, it can happen quite easily
3) to much smoke in general applied at once,, ,3 to 4 hours max at any one given smoke,, best to do 3 hours,,, cool / place in fridge,, ,over night,, repeat ..
hope this helps