Stalker: Freezing Touch has only 50% chance of Assassin's Focus

[ZemX]

As much fun as I've been having with Ice Melee on a Stalker, it's been nagging me that I don't see Assassin's Strike's lovely orange ring light up as often as I do on my other Stalkers.    I never bothered to check why until I happened to be looking at the power in City of Data and noticed it had only a 50% chance to add a stack of Focus.  Quickly checking in game, this seems to be the case.  It does not always stack Focus.

 

I looked around other sets and can't find a rationale for this.  Is it a bug?  Can it be fixed easily?  The chance to stack Focus seems to be related to recharge speed of a power and other 16 sec base recharge attacks, even ones with secondary control effects like Freezing Touch have a 100% chance to stack Focus.