Last Stand procs when stability is already up
I wouldn’t mind seeing some sort of visible recharge bar for traits like that, myself, so that I know in a split second whether to rely on Last Stand or to pop my utility. At least then it wouldn’t result in me using the utility, thinking LS is on CD, only to end up using both because of what you’re saying.
I believe defy pain does something similar, it will proc even if you are already under endure pain as long as you are under 25% and get hit.