Were all the images you were importing raw files? Or were there some Jpegs in there as well?
I don't normally use Copy as DNG but Lightroom has this way of switching 'Copy' to 'Copy as DNG' from time to time, without warning or notice.
I have never had this happen to me. If my last import was using Copy then the next time I open the import dialog it defaults to Copy. If I had Add selected then it defaults to Add.
Only if I use an import Preset does that change the default from the last import and then the default becomes whatever the preset specified.
This has been something that's plagued me for years in Lightroom. MacOS
Odd. I have never had this issue.
Can you see all files using File Explorer/Finder? Do they all show in Import dialogue box in Lightroom?
Just trying to see if there isn't anything corrupted.
Well, yes, obviously.