It's a 23andMe "problem" (it may be "huge" but it's not "fake"). There is probably a lot more Native American DNA in the US today than there was when Leif Erikson first landed.
Watch/read "The Last of the Mohicans". Uncas, son of Chingachgook, was the "Last of the Mohicans", because there were no pure-blooded Mohican women for him to marry.
There were no pure-blooded Mohican women for Uncas to marry because they had all married Dutch and English men.
(I have 23andMe, family lore, and a surname that's uncommon in Europe but common in the US, particularly in the Hudson Valley. Unlike Elizabeth Warren, I have never claimed to be a Native American on a job or school application. But I think the small % of my DNA that is Native is pretty cool, and something that's as American as apple pie. It goes against the establishment efforts to divide us all.)