When you say you set your default in Mail Settings I'm assuming you mean you went to SettWhen you say you set your default in Mail Settings I'm assuming you mean you went to Settings, Mail and at the bottom under Default Account you set it to Account C? If so go in and take a look to make sure it's still set that way.
Yes, that's how I set C to default and Yes, It's still set that way.
If it is try rebooting and if it's still misbehaving after the reboot switch the default to Account B and reboot. Open Mail and tap to compose a new message check if it opens from the default account. If it does switch it back to C and see if that sets it right.
Did all of this and new messages still come from Account B
However when you reply to an email someone sends you default account does not change the account the reply is sent from, that is determined by which account the email was sent to. Example someone sent an email to Account A and you click reply Mail isn't going to change your reply to be from Account C just because it is the default. The person sent their email to Account A so when you hit reply the email will be replying from Account A.
Actually, that's not the way it's working. As I said in my original post, no matter what address - A, B or C - an email is sent to, when I reply, that reply is always coming from Account B.