Yes, it's the lack of links that's the strange thing.
OK, they're light metal, and would corrode quicker than the heavier rounds, but even after 70 years, there should still be some evidence of the links. I've found links and empty cases (and live rounds) in the past, on various sits, dating from WW2 which, although rusted, were still identifiable as links.
Linking a round into a disintegrating-link belt is fairly easy, even by hand, but extracting a round from a link, or links, is more difficult, so for the rounds to become separated, even by explosive force, seems rather odd.