Hi,
I experience this from time to time with varying degrees of incapacity. I normally use the full width of a footpath and wait for the day I get caught being drunk in charge of the footpath, it is so pronounced. When I have a bad episode I struggle to stay upright with walking sticks, let alone walk. Had all the tests and bloods done with nothing discovered. MRI came back with low Sella and small vessel disease which could also impact on my walking. I do have Dysautonomia which likely accounts for it. I can be walking and have my left legs fail to take the next step and over I go, always the left leg. The brain has re-educated itself and I can usually catch myself with just a stumble. Fortunately I'm learning to live with it.
Cheers