as you can see in the tooltip https://i.redd.it/rukykpomur551.png the mantis is supposed to reset your cooldown for the decoy on a headshot kill and store that reset if it is active and not on cooldown but it has never done this for me on xbox one x. it resets the cooldown if it is not active but it has never stored that reset as promised in the tallent.
https://youtu.be/JiJvggm9oc8
It's on the Ps4 pro but the problem is exactly the same.
Already repported this bug on another thread.