Apple One - Apple
The answer to the last question, "How do I switch between individual subscriptions and Apple One subscriptions?" says, in part,
"If you cancel your Apple One plan at any time, you’ll have the opportunity to select any individual subscriptions you would like to keep, and you’ll be billed separately for them. Your subscriptions will all continue right where you left off — you won’t lose customized content such as Apple Music playlists or Apple TV queues."
When you signed up for Apple One, did you sign up for a Family subscription? $19.99 USD per month is the price of an Apple Music Family subscription. So if you were getting rid of Apple One Family, while keeping an individual subscription, it might have assumed that it should offer you the Family version of that subscription.
Apple Music - Apple