I had this happen - but I chalked it up to a punishment for using ad block, .. I got youtube premium for free when I bought Google Music (which is now youtube music?) ... it was a fair bundle for $10, all the music I want and no ads on youtube to boot ...
If they de-couple them I don't know if either is worth $10/mo indivdually.
Made me wonder if someone had found a loophole and was somehow making money from it?