iOS Mail added folders keep disappearing from Mailbox List

Anybody else having this problem?

In Mail, I select 'Edit', then 'Add Mailbox', choose the email account (I have 3 accounts, all MS Exchange or MS 365). Then select folders from my Outlook account, and finally select 'Done' twice. The selected folders appear underneath the standard Mail folders.

However, the folders I have manually added keep randomly disappearing from the Mailbox list, and I have to re-add them again. Sometimes they're gone within a few days, other times they go after a week.

This was happening on iOS 17 and continues since upgrading to iOS 18. I'm using an iPhone SE. It has also happened on my iPad Air (M1).

This behaviour has only started since the other two email accounts were migrated by my webhosting service to MS 365 accounts. Before this I never had an issue with disappearing folders.