This is likely either a coefficient of friction issue or a part that for some reason is not fit properly. That it doesn't happen right after cleaning suggests it is lubricant contamination, except you said it also happens with commercial loads? About the only contaminant then is carbon and soot from the powder. Some powders have quite a lot of graphite on them, and that can blow around and lube things some.
A couple of things to try: clean the bolt lug recess and locking block with Bore cleaner. That will soak into the carbon and let you remove it. Use an old toothbrush to get the traces off. In fact, if you can disassemble the bolt and block and soak them overnight, that's best. Wipe it as bone dry as possible with a clean, dry rag, like they sell at Lowe's or Home Depot for painters. Wear those nitrile disposables for painting so you don't get any fingerprint contamination. A micro-thin layer of the stuff will be left behind so you don't get rust, but the very thin oils don't have a lot of film strength, so they don't interfere with high pressure friction like grease does. The objective is to get everything as close to dry as possible without rusting it.
If you want a more specific recommendation, you could probably use Ed's Red for the carbon, but my favorite bore cleaner these days is Boretech Eliminator. It must be used with a plastic or stainless or nickel plated jag as it attacks exposed brass so fast the patch is blue from the brass before its gets out the muzzle. Eliminator has almost no odor, is water-based, and has a rust inhibitor in it
It is odd to see more bolt thrust as your gun gets dirtier, unless the graphite explains it? I'm thinking of work up to about 32 grains (easy to remember) of H322 with a 170 grain flat nose bullet. Like Varget, H322 has that sort of yellowish coating that Thales uses. Definitely not graphite. The Hodgdon site says 30.0 grains to start and 32.3 grains maximum. QuickLOAD thinks it could go a grain and a half higher with its default case capacity, but I don't want to speculate, and would advise sticking within Hodgdon's limits until you know otherwise with respect to your particular gun.
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