What you were expecting to happen, and what actually happened: Spell works correctly when 4 opponents, but when only opponents 2, 3 & 4, it only switches around 2,3, rather than putting them ‘to the back’.
How often does this happen? When did it begin
Every time you cast Giraffatur when the opponent has already lost their first troop.
Actually, it doesn’t seem to work normally when there are 4 opponents. It seems to move the 1st and 3rd opponents to the bottom, leaving the 2nd opponent at the top:
My best guess is that instead of moving the top two opponents “at once,” it moves the top opponent to the bottom, and then moves the NEW second troop (previously the 3rd troop) to the bottom. That would explain the odd behavior in both cases.
Also, if the spell is cast with only two opponents remaining, it switches their positions, which fits with my theory. Oddly enough, when the spell kills one of the two top enemies, then the spell seems to work as expected, moving the remaining enemy to the bottom. (The troop is actually spelled giraffataur, in case anyone cares.)