How can I un-flag iOS emails?

To clean up my emails, I flagged (using Rules, and four colours) various senders/addresses in Mac Mail.

I've now deleted all of the Rules, and...

• Flags have gone from Mac Mail
• Flags have gone from iCloud Mail
• But Flags are still showing in iOS Mail on my iPhone

I've rebooted the iPhone several times, opened and closed Mail, but 1,899 items are still showing as Flagged.

Any ideas why iOS Mail Flags are not being removed, and how I can make them go?

Thanks.