Move emails between Exchange and iCloud accounts on IOS

I have an Office 365 Microsoft Exchange email account in Apple Mail on my iPhone IOS 15.2 I also have this set up on two Macs (running 12.1 Monterey), as well as an iPad. The account is sending and receiving emails just fine on all devices. I have several other email accounts on these devices, all of which are iCloud accounts.

However, on iOS only (both iPhone & iPad), I can't move emails to folders on my Exchange account, from other iCloud accounts, because when I try, the Exchange account is greyed out. I have no problem moving emails from the Office 365 Exchange account to IMAP folders, but I can't do it the other way round.

I have no problem doing this on macOS – the Mail app on my Mac does it straight away, dragging & dropping, and those changes are instantly reflected on my iPhone & iPad. I just can't perform the task of moving an email from my iCloud account over to the Office 365 Exchange account, because it's not an option – it's greyed out in the list.

Even when the Mail app on iOS suggests a particular Exchange folder to move the email to (because I've done the same previously, with related content), it just says "Unable to move email" and it stays in the IMAP Inbox.

So, it works on macOS, but not on iOS. Is there a setting in IOS that is not correct?