Hello Samir
The command show ip bgp neighbor x.x.x.x advertised-routes shows the routes that are being advertised to a neighbor before any outbound policies are applied. This is why you don’t see the changes made by your route-map for AS path prepending.
However, if you deny a route using a route-map, it will not be in the list of advertised routes because it’s not being advertised at all.
To see the changes made by your outbound policies, you can use the debug ip bgp updates command. This command will show you the updates being sent to your neighbors, including any changes made by your outbound policies.
I hope this has been helpful!
Laz