Running iOS 18 beta on iPad 9th Gen.
I have two email accounts set up in Mail - an iCloud account which is the default account in 'Settings>Apps>Mail>Default Account'.
I also have a Gmail account - which gets most of my mailing list, digests and general 'junk' etc.
I use the Mail app & the 'All inboxes' view to read my emails on my iPad.
I want to send all new messages from my iCloud account but, despite it being the default, this does not happen.
Whenever I start a new message, the sending account selected depends on the highlighted message in the left column. So, when I start a new email to my wife, if I happen to have a mailing list message selected from my Gmail account, the email gets sent from my Gmail address. Unfortunately I have more mail sent to my Gmail account than my iCloud so it is more likely that the non-default account is chosen.
On macOS, the default sending address is honoured whatever email is selected.
This is inconsistent behaviour between the two systems.