In such a delicately flavored beer, dissolved CO2 -- that is, carbonic acid -- probably comprises an outsized proportion of the flavor balance. Since they expect American lager to be served very cold and fizzy, the brewers may be formulating it to taste right under just those conditions. As mentioned above, as it warms it loses CO2, and I suspect this shifts the flavor balance, losing the element that, in the absence of high hop rates, counters the sweet flavors of malt, grain, and even alcohol (which can be tasted in a very delicate beer.) It's more or less the same thing, in reverse, as serving a British ale cold and overcarbonated. Possibly. And it might be crappy beer to start with. I don't care for the B, the M, or the C, but I do enjoy PBR.
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