2928 Outgoing Routing Issue

2928 Router using the latest firmware (5.4.4.4, but also was an issue with 5.4.4.2).
I have a Route Policy set up for one of my internal IPs (10.x.x.x) to use an alias from my public IP range, but outbound traffic still appears to come from the main router IP.
This was working previously - has anything changed, or is there something I've overlooked?

I've just tried connecting a PC via one of my IP aliases and found the same as you - What's My IP reports the main router IP address. I can ping the alias addresses from outside so they appear to exist, but something ain't right!