iPhone mail ignoring IMAP path prefix after 10.3 upgrade

After upgrading one of my iPad mini's to iOS 10.3, the mail app no longer uses the IMAP path prefixes on any mail account I have. (I have five mail accounts configured, of which two use path prefixes.) My un-upgraded iProducts continue to work normally, so I can't upgrade them until this is resolved.

Has anyone else experienced this after the iOS 10.3 upgrade?

I've tried rebooting the iPad, deleting and re-adding the accounts, even setting the path prefix to /tmp to see if anything was being used. Nothing has worked. I know similar issues were discovered in the past, though I'd never experienced them. Is this a new issue, or the reintroduction of an old issue?