Thanks so much for the advice @Diseasel300.
I've checked the nozzle seat in the lower half and cleaned with a wire brush. There was no rust and no obvious carbon build up. It is now soaking in thinner.
I did notice that this surface is not flat/level. It seems it is 'stepped' down from out-to-in. I can feel it with a small punch/point.

I checked one of my other 'good' injectors and it too is not flat, but seems to be less pronounced than my 'problem' injector. Ive tried this injector with 2 different new nozzles and the same problem plagues both. I'm hopeful the soaking and another good cleaning will do the trick...but I've already ordered a Bosch reman injector to be on the safe side.