Thanks for digging this up! It only addresses the YouTube half, but if memory serves the Google half is due to a mandatory deep integration between browser profiles and Google cookies. I'm unclear if they classified that one as a "bug" (not that it really matters since they use custom HTTP headers to implement tracking for google properties).
Not sure what the sibling comments are talking about. The bug fix[1] links the original change that introduced the bug[2].
[1] https://chromium.googlesource.com/chromium/src.git/+/f800edb...
[2] https://chromium-review.googlesource.com/c/chromium/src/+/21...