I have a personal and joint account, when I go to the payments screen it seems to be random whether the account selected is my personal or joint.
My wife has found the same thing and is always sending personal payments from the joint account by mistake.
Is there any logic to which account is selected and ideally is there a way to set a default?
If you’re viewing your joint account and then go to send a payment, it will have the joint account selected.
Likewise if you’re viewing your current account, and do the same, it will select this as the payment source.
Not for me 
Just tested and whether I was on the home screen (both accounts combined), personal or joint it had my personal account selected when I tapped the payments button in the nav but other times I will go to the payments screen straight from the home screen and joint will be selected.
My wife never selects to view the joint account and always goes to payments straight from the home screen yet joint is usually the default one selected for her
I can confirm this is how it works with mine. I think it just stores your last viewed account in the payment screen.