Gentlemen, please consider that both ANSI and NEMA spec 1000V meggars as proper for field testing of used induction motors. Also, consider that a brand new non-MG1 Motor is tested with 1200VAC at the factory and an MG1 motor is tested at 1800v DC pulse power.
Testing a motor with an insulation system designed for 480VAC at levels above 2000V can be expected to damage perfectly good insulation and convert a good motor to bad simply by testing it.
Finally, expecting to see 1000000 times the nameplate voltage as the lowest acceptable meggar test resistance is way off. It isn't unheard of for a new, bone-dry motor to test less than that.
The general rule I've seen most commonly at 480VAC is at least 1000000 ohms to ground and, when possible to test, phase to phase. Less than that indicates a motor recondition is needed and maybe even a rewind. It depends how high the resistance goes after a good cleaning and bake-out.