If waze says one route lasts 48-53 minutes, this is an average of all people. Could be that you personally would be much faster. But waze uses the average. In most cases fast drivers beat the ETA.
What is the real difference in time? Try driving both routes and have a look at your watch. 10 minutes is a huge difference for a route of about 50 minutes. Try that multiple times to have an average. Only one time 10 minutes faster does not count
What if you take your route. Is there a point within the route when waze switches to "your" preferred route? How does ETA change during that switch?
Maybe the map does not support your way? missing turns or anything.
I wonder what Google Maps says to both routes. You know in Gmaps you can configure individual routes by just dragging them...
You know, there can be numerous reasons for the situation :/ On one hand it could be that waze still don't know your way is faster. On the other hand it could be waze still don't know the other way is slower.