1986 190d non-turbo 602

I had my injectors tested, and replaced a couple along with all new heat shields. I cleaned all surfaces well and put things back together. The engine runs great with good power and relatively little noise, except:
Cylinder 3 produces a compression leak through the threads of the injector body- between the injector and the injector holder. It doesn't leak around the outer threads of the holder, nor around the prechamber collar/head threads.
I switched injectors and heat shields, and retested several times using soapy water. Injectors that don't leak in other cylinders exhibit the same leak in this hole.
It's definitely compression, and definitely only between the injector and its holder. And definitely only on this cylinder with injectors that don't have this leak elsewhere.
Thank you for your expert help, this has me baffled!