Check the headspace with a go gauge. If this comes out to be OK (amd I'm fairly positive that it will), then I have little doubt that the bullet is engaging the rifling (jammed) too early probably due to the freebore being too short (at least for the "stuck" rounds). Fed. GMM uses a sealer on the bullet in the neck. My guess is that the leade is so short that the action of jamming the bullet into the lands was sufficent to "break" that seal and thereby allowing it to be pulled from the case when you pulled back on the bolt to remove the cartridge.
Should be easy to determine by using your Hornady OAL gauge and find the length to just touch the lands...
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