I think having to constantly reset stance is a pretty silly, distracting, and unnecessary part of the game play experience.
I think, Dulrik, if you want the prone to have the additional affect of taking you awhile to get back into your stance, simply code it so your stance AUTOMATICALLY readjusts at the end of combat. Perhaps (another thought), it should reset at the next tick after combat, if you wanted a small lag. There's 2 ideas to improve what you've done.
I'm not saying when you get knocked prone you shouldn't lose your stance. That's kind of a nifty idea really. I'd have to play test/be play tested on to see what sort of a difference this change is otherwise making. I AM saying having to constantly reset stance is a pretty frustrating thing, especially considering how many times/ways a character can be knocked prone.
Considering also that triggers are forbidden, what exactly were you aiming for here? 100% of the time a stance is lost a player (should really) reset it as soon as they are able, RIGHT?
Please reconsider.