I assume your system is in the CST time zone based on the 6 hours difference from GMT. If you look at an event and say it starts at 10AM CST or it starts at 4PM GMT is equivalent, there is no ambiguity. Editing the event in either GMT (changing from 4PM to say 4:30PM) or CST (changing from 10AM to 10:30AM) is exactly the same, it is a consistent change. Are we in agreement on this point?
Now, you have a valid point of view namely "...the times displayed when editing the time of an appointment should be in the times you entered, not in GMT time..." but it is no more and no less than your point of view on how you expect the system to behave. I may have a different point of view which says "...events will be created, by default, in your local time zone, all subsequent changes to the event time will be performed in GMT..." you must agree that it is a viable point of view, even if you do not like it.
Unless we can reach an agreement, which we will specify and document in a design document we can not, we can not decide what is a bug and what is not since we have differing reference points. Since we are not privy to the Apple design documents there is no way for us to tell if this is a bug or not. Only Apple can make that determination since they specified and designed and implemented the system as they thought was desirable.
The fact that the system behaved differently prior to the iOS 8 update makes it somewhat suspicious this change in behavior was intentional, but nonetheless it may have been for reasons which Apple has not communicated to the public. You must give then this benefit of doubt.
There is no absolute here which by which one can say this is a bug or not unless we have access to a common and agreeable reference.
Having said all of the above do I think that the behavior of the calendar past the iOS 8 update is desirable, I am definitely agreeing with you it is very bad, it is annoying, it is inconsistent, and Apple better listen to the outrage in its community of users over this change. For them to stay quite and not publish an intention to fix this problem in a future update to iOS is disrespectful of their customers, in my opinion. Nonetheless, the system is working correctly even if in an annoying manner. This is my only point, it is not a bug. It requires a change in requirements. Please read mikalh's post of January 27, 2015 who was the first as far as I can tell to argue in this discussion that this is not a bug.