I don't have premium and it's sticky for me but only on a single computer, I have to reset it if I switch computers or browsers. Same with dark mode. So maybe it's stored as a cookie and they wipe their cookies?
Surely you don't expect YouTube, a company that doesn't store any data at all actually, to be able to store a single boolean value somewhere in your account, do you? This would be impossible for a company as broke and small as YouTube.
