Take some of your pink insulation out, light up some incense. If the incense smoke it pulled out of the house then your build is wrong on the inside. If the incense smoke is pushed back inside then you need some kind of external chimney.
In all the rocket stove u-tube videos I have seen I don’t think any of them exhausted above the roof. I guess I could at least try hooking on a vertical 6 foot flue pipe on to where the flue exits the home to see if that works. That would be easy enough.
I had the SAME EXACT problem as you, for me the solution was hooking it up to an external chimney and all my problems went away. I had outside positive pressure fighting RHM positive pressure.
See my pics My build