@Starman
It sounds like the rig itself is confused on how the knee bends. If the model had no IKs to direct it or had a bend when it was being rigged where the knees are, it gets confused and will automatically decide which way they would bend.
I have seen this issue with many models and I myself had it with one a long time ago. The only solutions I know of are either going into VRC FBT settings even if you aren't using FBT and go to the area where you can rotate joints. I believe there is one for knees, but I could be wrong and just rotate that until the knees bend how you want them to.
The other one is going into Blender and bending those knee bones forward a tiny bit and reuploading it to VRC.
Those are the methods I personally found through experince.
It may not be the solution you're looking for, but that's all I know.