Not forgotten! Both forms have canonical equivalence but they’re not identical as you say. Hence why its quite important to normalise to :nfc before checking casing for consistent results (unless implementing a full casing algorithm that is normal form independent). By the way, not only an issue for diacritics. Also for hangul. And not all decompositions are canonically equalivent.
For example half-width and full-width katakana characters will have the same compatibility decomposition and are thus compatibility equivalents; however, they are not canonical equivalents. They also aren’t cased so at least that’s not an issue here
