Well, today i was doing this leve...attacked the first couple. They turned into their true form and then...

"the darkwing devilet's group is engaged."

We killed the imp then fled from fire elemental...but when we got far enough...

"the darkwing devilet leaves combat."
"the fire elemental returns to its territory."

So i'd assume one of these errors might have happened:
a) It was meant to be "the darkwing devilet's group leaves combat."
b) or "the fire elemental leaves combat."

For reference, when i attacked a couple that did not transform the messages were:

"the fire elemental's group was engaged."
"the fire elemental returns to its territory."
"the fire elemental returns to its territory." (2x because there were 2 eles.)
"the fire elemental leaves combat" (only once...so i'd assume it refers to fire elemental's group, like the engaged message, and not to fire elemental monster.)