Anyone else have Boris's stalking stance fail?

I tried using the ritual that grants one of Boris's armies the stalk stance, but when I use it to attack enemy armies it only does a regular attack instead of an ambush despite the ritual claiming that the ambush is guaranteed.

Has anyone else had it work correctly in their Middenland campaign?

Edit:

One of the things that suggests to me that its a bug, is that after I back off from the attack, the game switches the army back into normal stance automatically. I dont remember this happening to any armies from factions that normally have the stalk stance.