I had a similar experience with Google Photos where it merged the "profiles" of my two children. Like you, I tried separating them back out manually, with no real success. Ultimately, I turned the face identification feature off entirely (which has the effect of deleting all of the face data), and then turned it back on. It took a day or so for Google Photos to start re-indexing the photos in earnest, but that fixed the issue for me, and it was less work than the manual re-tagging that I had tried before.