I've seen this issue, too, and there are lots of reports of similar problems on Apple's discussion boards. I think it's a bug with the iOS, not with Fastmail.
I've noticed that restarting the iPhone clears it out temporarily—it's probably forcing Mail to refresh all its info from the server. You can also force just Mail to quit: From Mail, hit the Home button to go to the home screen. Double-tap Home to open the app switcher. Tap and hold on Mail until it start jiggling and shows the red minus badge. Tap the badge to quit Mail. Hit Home again. When you next launch Mail, it should show correct information. Temporarily.
/jgt
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