1/16 is very bad. Wrong length bolts. If your measurements are off 1/32 on each piece or bolt length tolerance, you will bottom out, bad bad bad. Very bad. Holes could be different depths. Not familiar with that. Betting you snapped the bolt because it was bottomed out.
Over 2 1/2 length, the tolerance is +.06/-.08. That will bite you. A bolt maker will not shoot for the minus side. If the threaded hole is a blind hole, it probably DOES NOT have thread all the way to bottom. Even using a bottom tap. And you may be measuring depth to the point of the tap drill. That will throw you off with a false depth.
You don't need a massive amount of thread engagement. You do need 1/4 inch gap at bottom, minimum.
If bolt and mating threads are same material, 4 threads engagement is as strong as the bolt. Some designers have a 1D engagement standard. (in your case: 1/2")
