This is going to be tough to diagnose in the absence of seeing the code involved. We probably need to start with seeing this “certain form”, and the view that is processing it.
Thank you very much for your kind offer to help. I figured it out!
See
Apparently this is a “feature” added around ver 1.9! The logic, I suppose, is that
if someone modifies their user object, then it may be good to quickly verify you
can still log in. The link shows how to override it too if desired.
I suggest, for your own benefit, to never rely upon StackOverflow for accurate information about Django without referencing an authoritative source, nor to post links to it as references here. (SO is an “ok” starting point, but should never be considered authoritative.)
In this particular case, the information you’re looking for can be found at: