Hey there Bob, I'm sorry to hear your users are having issues with this. I don't personally use IMAP on my iOS device so I'm not sure how this was handled in the past, unfortunately.
I did set up a quick IMAP test on my device though and found I too see the Sent Items selection reverting back to the original selection. Can I have you submit a ticket on this one so we can dig into it further together?
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com