https://duckduckgo.com/?q=valheim+floating+copper

Most likely:
It's not floating because as long as any single piece/chunk of one copper node is touching ground "the whole node is not yet flying".
They only fall apart on their own when they are completely disconnected, but some pieces can be below the ground - lower than you can dig.
This ^^ happens especially often when the copper node is already on a slope and not on flat ground (my observation).

I think you can even test this with the Stagbreaker for example.
Just like you can find buried scrap piles and silver veins without the wishbone.