This question is better suited in a support ticket
This forum is the third party developers forum, mostly consisting of community members, and we don’t know about how Twitch handles chargebacks, only Twitch itself does. So you’ll need a ticket
I imagine since transactions go between the user and Twitch, (and not you) there is never any fees you pay if a chargeback occurs. And a chargeback on a subscription isn’t as simple to do, compared to one off payments (that occurs with tips which is between the user and you)
I think Twitch only says “bits only” as “bits” are a form of Tip and those are “straight forward” to charge back as they are not exchanged for anything.
Where as a Subscription, you exchange Money for a recurring subscription to a service, and thus you can prove that service has been used, which’ll often cause the chargeback to be rejected.
Again just my opinion there, from my previous experience running similar stuffs