A properly gassed 16" midlength gas system could short-stroke with some ammo when using a H2 buffer, especially with a plus-power spring. Those generally run best on heavily overgassed systems.
Just because you try different weight bullets is near meaningless. Different powder burn rates have more impact on this than bullet weight, and you don't have any control over factory ammo.
I'd switch to a standard carbine buffer, or H1 weight, and see of this behavior continues. I'd also spend a lot of time single loading to test locking back every time.
Edit to add, you didn't say what you were using for lube. Make sure you are using CLP or something similar, lightly lubed in the important places, and not using any grease or froglube in cold climates. (lets not start a pro-FL or anti-FL discussion please)