Weak plantarflexors could absolutely lead to uncontrolled tibial advancement and, therefore, knee flexion during stance phase.
They could also cause knee hyperextension. Individuals with profound hamstring and/or plantarflexor weakness may have knee hyperextension in mid-stance due to a lack of eccentric control of knee extension (i.e., normally functioning hamstrings or plantarflexors could prevent the knee from snapping into hyperextension).