I have managed to resolve this issue thanks to my colleague Rich, who linked me to this answer
I’ve recently been using an iPad much more frequently than I have in the past and during this time my frustration over swipe actions in mail has grown into all consuming anger.
Swipe actions in all apps but especially when triaging mail should be the simplest, muscle memory function that can exist with touch devices. Instead I have to double check I am swiping the right way every time.
For my iPad, I have to swipe right but on my iPhone I need to left.
As you can see both devices are set up to archive on swipe right and both are linked to the same fastmail email account.
Here you can see that the results are different.
iPad swipe right (correct)
iPad swipe left (correct)
iPhone swipe right (incorrect)
iPhone swipe left (incorrect)
I have searched and searched but not been able to find the answer. Please contact me if you know the answer.