It's pretty hard to tell from your video..
but here are a couple things I'd check.
first I would just check if there are keys on that knee control ( I'm guessing not)
next I'd arrow up to the knee controls parent. and see it that moves ( or is driven by something)
*( and continues walking up , until you find whatever is moving that knee control)
*(it could be a matrix parent connection. so it might not be obvious)
Next ( and I'm guessing this might fix it based on your video)
is to change and test your animation evaluation mode: from DG, serial, or parallel.
It may be that whatever controls that knee position only works with a specific eval mode, and not with another.
I'm assuming this is a free rig you got somewhere? (merry rig ? )
Maybe reach out to whoever created it , and see if they know of the issue, and how to fix it.
if none of that helps, then maybe you can share a file
hth
-=s