Probably by design, but the echo could use changing if it is.

I killed this guy, but unfortunately the weapon I was using wasn't the unholy blessed one that I thought it was. Unsurprisingly, no charge.

Some time later (maybe a day, if not, at least several hours later), I kill him with a properly unholy blessed weapon. The echo I got tells me that my weapon had already fed on his soul, even though it was, I think, the first kill I'd actually made with an unholy weapon in hand.

Also, does this mean that the first one, where I didn't have an unholy in hand, would have counted towards controls, but the second one would not have? That feels difficult to explain if so.

n/t

But that still leaves the incorrect echo. It says my weapon has already fed on him, when it hasn't.

Also, I'm not sure that killing someone a day later (or even if it was same day, but 6 hours later), is necessarily a multi-kill. But I'm quite happy for you to continue to class it as such.