It could be part of l’hermittes. Sometimes chin to chest gives us an electric shock type feeling, and often when I had it it would bypass my mid section and go straight into my leg or legs. I remember years ago having it and falling over in the garden. That was back when I could easily pick myself up (ah them were the days!)
At other times, I’ll be walking (um slowly with FES and a walker!) and one knee will sort of ‘fail’. It’s like the ‘hinge’ kind of cracks and the leg semi gives way. If it happened to both legs simultaneously I’d probably hit the floor. But so long as one leg stays firm I’m still upright.
So yes, to me it sounds like an MS kind of thing. Obviously if your legs are generally quite strong, the physio probably isn’t going to give you much in the way of help. They tend to know about muscles and obvious things, but nerves suddenly making knees fail is possibly a bit beyond the knowledge base of the average physio. But maybe just make sure your quads stay strong, then your non failing knee will hold you up.
This is obviously information based on my experience. It may not be the same thing that’s making your knee fail. Maybe mention it to your neurologist if you have an appointment soon (or I’d be tempted to write and ask if you don’t expect to see him/her soon).