Route to entry in Arabic not working but category route is OK
So it looks like I made some false assumptions.
In Chrome the Arabic URLs displayed are reversed in segment order.
However, I assumed the same was true for Craft routing - the UI behaves in a RTL fashion when editing Arabic text in routes editing interface. But this is not true - it seems that Craft routes don't reflect the browser behaviour for RTL languages. Craft still keeps the normal order of segments when processing the route. So weirdly the URL is reversed in browser but Craft seems to 'see' the route match as left to right. Once I adjusted the route to 'normal' left to right segments the routing worked fine.