OK, so you have two different interfaces with IP addresses in the same IP network. That configuration only makes sense if both interfaces are connected to the same physical network, and under those circumstances the behaviour you're seeing is actually expected.
As this article
explains, the reason is as follows: When the Linux IP stack receives an ARP request for an IP address assigned to an interface, the ARP reply will be sent from the interface that received the request, using the MAC address of that interface.
This behaviour is perhaps somewhat counter-intuitive, but it seems to be a valid interpretation of the relevant RFCs.