Do you have a citation for that?

From what I understand (and years of use) they merely place a small amount of weight on the front of your shins, which is at most merely uncomfortable for some people. (Most of your weight should be supported by your bottom.)

For an average human body, I have a hard time imagining what biomechanics could lead to a knee issue from it? A quick Google search didn't show anything.

Sadly, no. It's based on discussion with healthcare professionals (physiotherapists, orthopedic surgeons, ...)