However, there's plenty of evidence suggesting that there wasn't any such regionally adaptive behavior. The original find that suggested starchy plant consumption in H. erectus was analysis of the Dmanisi fossils at ~41N. Later work tying these sorts of results to typical European Neanderthal diets has been done for North Sea sites (~50N). Of course, Neanderthals never went much farther north than that due to the climate. As for AMHs, I've seen papers of sites near Smolensk (~55N) indicating moderate to heavy starchy plant use. That's not far from the glaciation line.

Moreover, there are fairly convincing arguments that a nearly pure-meat diet doesn't work in winter due to the general lack of fat (e.g. https://doi.org/10.1016/j.quascirev.2009.11.016 ). There are caveats and workarounds here, but this whole idea that high-latitude hominins had limited, homogeneous diets is on increasingly shaky ground.