You are entirely sure it is seated and torqued properly?
I've replaced and installed lines and injectors a few times over the last 20 years.
Threads Ok? Cross stripping on the nut or the top of the injector?
Interesting issue. Never seen one leak so incessantly.
I have a hard time imagining a failure or void there. Those bodies seem to be tool grade steel or perhaps hardened.I'd put the injector on a pop tester and see if it really is leaking there or a maybe flaw where the return barbs meet the body.
It's a gamble. I'd make sure you have a spare that you can swap in incase everything goes wrong.
I wonder if its a leaky return line