Older Gibsons were built to include a bit of 'belly bulge', or in other words the tops were not braced to be completely flat under tension even as new instruments. Still the case with Gibson flattops from Bozeman IME. Every Gibson I've had has had 'belly bulge'. My HD-35 and a host of other Martins had the same thing, from vintage to newer stuff.
Too much is too much and I've rarely encountered it except in a guitar that had loose braces or something else going on.
'Top sink' = the top sinking above above the bridge toward the soundhole also happens but is not planned into the build and generally not a good thing at all compared to 'belly bulge'.
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