I don't have a problem with my inner knee on the front leg, but I do occasionally have problems with the outer knee on my front leg (iliotibial band).
I also ride 15/-15.
I notice, at 21" and 15/-15 (I'm 5'6" but with short inseam) I also put a lot of pressure on my outer front foot. To the point it starts to ache, get fatigued, burn.
I've only been out once, but I lowered my angles to 12/12 and had a good day and notice I don't press on that front outside portion as much.
Try to bring it in for a day. Or bring it out. Try different stance widths.