Currently, subscriptions that haven't been manually cancelled attempt to auto-renew approximately 6–10 days before the subscription expires.
This would be fine, except that when the payment method charge fails, even though there is still 6–10 days of subscription left, the sub becomes inactive and begins showing "change/fix your payment method to continue your sub" messages, while showing ads and losing access to subscriber bonuses like emotes for the channel.
On top of this, when you do try to fix your payment method for channel A, if you also have expired subs on channels B, C, D, when updating your payment method, it attempts to charge for ALL of them, even if you only wish to resubscribe to one. It doesn't specify an order of operation on which channel will be charged for first.
I have "updated my payment method" to the same card that is already being used, after putting just enough money in the charging account for the one subscription, specifically from the button on the channel in question, only for it to fail, because that money was spent resubscribing to some other channel I haven't watched in months.

I keep my charging account empty until I specifically plan to spend money. For a few months, I have shrugged this off because I thought I wasn't paying enough attention and was in error. So last month I marked on my own calendar when I purchased any subscription, and then marked when the subscription benefits ended, and I was asked to fix my payment method.

Effectively, you are cheating subscribers out of roughly 1/3rd of the promised purchase value. And adding a process of fixing the sub that is annoying, obfuscating, and wildly misleading. It is the sort of thing people should be emailing the ACMA, TIO and ACCC about.
Either you need to change this method, or you need to start advertising subscriptions, at point of purchase, as "approximately 20 days" rather than "1 month".