I believe the fundamental problem, affecting IMAP, is that embedded graphic content, or email attachments, become randomly unviewable unless you have an active network connection. This becomes a problem when attempting to access mail, that has already been downloaded onto your iThing, when you're offline. The ability to review already downloaded email - and draft replies - is a basic requirement for offline Mail. Unlike webmail, you should not require network connectivity to review material already retrieved from the mail server.
In defence of those that claim to see no problems, I suspect that they might only be accessing email when they have a good, solid, reliable, continuous network connection - be that WiFi or mobile operator. I too can create conditions whereby I see no issues, but in the real-use world, my email is reliably and fundamentally broken if the network connection is interrupted. My iPad is with me round the clock, regularly switches between network connections, and is frequently beyond network coverage. Already downloaded emails, other than simple text-only stuff, ceases to viewable unless a network connection is present.
If Apple have deliberately changed the native iOS mail-client, such that it now requires an active network connection at all times (which, for example, cannot be guaranteed when travelling), then they should be open about this change in functionality. Conversely, if this is a bug introduced with iOS 9.x, then some effort should be expended by Apple in acknowledging and fixing the issue. Unfortunately, neither appear to be the case.
Have you also noticed re-appearance of another, previously solved issue - where the first two letters of a sentence are automatically (and incorrectly) CApitalised - and sometimes with an unexpected line-break?
