On the cylinder test the cylinder threads are checked and the mating valve threads ie m25x2 or 3/4bsp etc not any other threads that do not make up the valve to cylinder seal < The 5/8bsp din fitting should not be touched and as far as i am aware is not required under the relevant standards.
If the valve was a special with some unusual thread on the filling/outlet port ie unf/unc as long as the actual valve to cylinder thread gauges up then it has passed but they cannot fill to test the cylinder/valve seal then both the cylinder and the valve should be returned to you unfitted as the hydro test only applies to the cylinder.
I tested many thousands of cylinders working for MDE and that was the set procedure then.

If you can do it take the valve out of the cylinder and just give them the cylinder to test.