iOS 26 Mail App Showing Messages In Incorrect Folders: iPhone 15 Pro

Since I upgraded to iOS 26 my mail app is showing mail in the incorrect folders.

For example, I raised 3 support emails with a third party yesterday (not using the iPhone, using my non-apple mail app - Thunderbird).

On the iPhone mail app my Inbox shows 2 of the messages that should be in my Sent folder and one correctly showing the auto reply I got from the third party. These also aren't consistent with the previews of the emails.

My Sent folder shows the same - 2 messages that should be in the Sent folder plus one auto reply from the third party that should be in Inbox, although they are not the same emails as displayed in the Inbox. Previews are also inconsistent.

My Thunderbird app shows all the messages in their correct folders.

I have switched off the following in Settings->Apps->Mail

Summarise Message Previews

Show Contact Photos

Show To/CC Labesl

Followup Suggestions

Everything in Threading

Preview is set to 1 line

I am in List View

Can anyone suggest what is going on and how to remedy this?