hey, i know this is really late but have you tried going into the mail app, backing all the way out to the Mailboxes screen and then searching from within the individual account at the bottom of the page? I came here looking for an answer to the same problem and, in futzing around with my phone, tried the search there instead of within the "All Inboxes" folder that i use all the time. When i searched in "All Inboxes" i had no luck but when i did the same search from within the specific account I got the right results, maybe a dozen, from the 33K emails it searched.
Shame it doesn't seem to work from All Inboxes because that's usually where i leave the app. Would be curious to know if the same problem would exist if there was only one account on the phone. i have three.
Hope that helps somebody.