Having to transfer a money every month from a joint account to a personal just to pay for the upgrade is a really pointless exercise.
I get having it tied to one account, it earns Monzo more money, but the payment should be able to come directly from the joint.
My personal account is literally used for nothing else but this.
Schedule the payment from a Pot in your joint account to your Personal account the day before it’s due, problem solved, no effort involved.
Welcome to the community @WastelandStag. 
Extra/Perks/Max is attached to your Personal account only. Yes, it gives you some of those benefits for use on the Joint account, but only YOU can use them, not the other person. Since it is YOU who has the extra features, you need to pay for it from your own account.
(The insurances that come with Max are also only for you. Max Family is still for you and upto 4 family members at the same address. You can open a Joint account with anyone, even if they are not at the same address, so the insurances won’t cover them, even if you had Max Family).
Agree again and as with overdraft and joint Flex being payable from joint account, the insurances should be this same, payable from joint account. I am not entirely certain Monzo live in the world some of us do in reality and do not understand why they do not allow these options. Maybe they will soon…