Are you even asking if it's a bug?!
In offline mode, it's definitely a bug, and it shouldn't be used; it's a violation of the rules.
But for online play, this action cannot be considered a violation (except for Global Overload), as there is no replaying in online mode, and you cannot know if there is a "greened" stack there or not, when you don't know if you will win the battle or not (in the first week, this is a fair statement; I think no one will argue that it's okay to lose). After that game (with the ultra-butterflies), I immediately found myself in a similar situation: I had enhanced and non-enhanced neutrals left – I didn't make my hero run away again (there was no point), and yet there were fewer neutrals. So, explain to me where the BUG is here (I don't have an army from the first week; everything was lost). And what if I get a technical defeat – who would dare tell me that???
My opinion:
A bug is an intentional violation!
Regarding Global Overload, it's a bit more complicated; I would slightly adjust the rules for inter-resource tournaments:
It is forbidden for a hero (or heroes) to repeatedly attack and retreat from (or die) a non-greened Global Overload more than once per game if it has a greened stack. - but I would allow the main hero to attack it!