I think I see what's going on in fact.  The cycle lane along High Road Leytonstone isn't continuous and the routing takes the user on to the pedestrian pavement, I think because it doesn't realise there's a continuous cycle way that just goes straight when it gets to intersections.

What's the proper way to deal with this?  Just set the cycleway:left and cycleway:right on the whole road even if it momentarily disappears while crossing intersections?