Ditto on what SAMHOP said on post #37. Some valve bodies are made with straight tread (NPS) and use a fitting with an o-ring and nut to seal.
Example:
https://www.worldwidefittings.com/en/90-straight-thread-elbow/
This fitting allows your to align the fitting in any rotation and seal it in that position.
Putting in an NPT fitting sometimes works but can often crack the casting.
Did some work with stainless fittings on medical drug production equipment, and was taught to always use tape and dope together. The tape must never go nearer than the first 2 threads of the male fitting to make sure it never got into the piping. If you took a joint apart you had to be sure all old tape was removed before re-assembling the joint, with new tape and dope.
Also, some valves used flared or sleeved metal lines directly connected, think of the brake lines on your truck or car.
Did you look carefully into the threaded hole and notice if the threads were straight or tapered? And was there any seat area machined around the hole?
If it leaks again, please post a clear photo of directly into the hole with the fitting removed.