Hole inside fork's upper tube, air-spring side?
I don't think that's a manufacturing defect.

Common manufacturing defects of suspension forks (though defects in general are not common) are defective oil seals and/or air pistons, improperly peened/stamped steerer tubes/stanchions, inadequate fluid (oil), and general machining problems such as caliper bolt threads.

Very very very rarely would one mistakenly drill a hole through the bottom part of a stanchion.

Now, your fork is a Rockshox one, right? If I recall correctly, some systems of RS forks (DebonAir, I think) automatically equalize pressures between the positive and the negative air springs after pumping up by cycling the fork through its travel.
That hole is most likely an air port utilized for such systems. (RS forks used to have a feature to separately pump each air spring to your preference.)

If you're really that worried, you can contact Rockshox themselves and have them sort that out for you.