Darius Ult Bug

I had this weird interaction earlier with Darius ult where it had seemed that I got the kill with ult considering the cooldown reset. However I did not get noxian might. I flashed forward for what I thought was going to be a sick quadruple decimate with noxian might only to be disappointed when i didn't see 4 giant axes after decimate landed.

At first I thought it was just like if the ult didn't land and the CD was refunded. But then I noticed that the mana was not refunded and it also looks like the ult lands. To further add to the questions, it looks like no mana is spent when using the ult a second time as if it had been reset by the guillotine kill.

Bug or working as intended?