Packets sent from an alias have wrong source address

I have a private IP alias on my public ethernet interface.
The problem is that packets sent to private addresses have public source
address instead of the private one. This behaviour for example breaks the
firewall rules.
The box does not act as a router.

$ ifconfig eg0
eg0: flags=8c15c43<UP,BROADCAST,RUNNING,FILTMULTI,MULTICAST,CKSUM,DRVRLOCK,
LINK0,IPALIAS,HIGHBW,IPV6>
inet 150.254.173.5 netmask 0xffffffe0 broadcast 150.254.173.31
inet 192.168.128.14 netmask 0xffffff00 broadcast 192.168.128.255

$ netstat -rn
Routing tables

Internet:
Destination Gateway Netmask Flags Refs Use Interface
default 150.254.173.14 UGS 6 18959 eg0
192.168.10 link#5 0xffffff00 UC 0 0 ef0
192.168.128 link#4 0xffffff00 UCS 0 0 eg0

Here is what happens when I try to ping, say, 192.168.128.1:

# snoop -V 192.168.128.1
Using device eg0 (promiscuous mode)
________________________________
cactus -> 192.168.128.1 ETHER Type=0800 (IP), size = 98 bytes
cactus -> 192.168.128.1 IP D=192.168.128.1 S=150.254.173.5 LEN=84,
ID=54202
cactus -> 192.168.128.1 ICMP Echo request (ID: 7661 Sequence number: 0)
________________________________
192.168.128.1 -> cactus ETHER Type=0800 (IP), size = 98 bytes
192.168.128.1 -> cactus IP D=150.254.173.5 S=192.168.128.1 LEN=84,
ID=63556
192.168.128.1 -> cactus ICMP Echo reply (ID: 7661 Sequence number: 0)

Can it be because the primary address has a different netmask (it is
recommended at techpubs that "the primary address of an interface and all its
IP aliases should share a common subnet address.")?

Thanks for any help,

-marek