ASA transparent mode subnet mis-understanding

Hey all, trying to get transparent ASA deployment down and I'm hitting a road block concerning something that I feel should work, but does not.

Setting the ASA with ROUTER(subnet-A)------------ASA(subnet-A)------------ROUTER(subnet-A) works just fine. Traffic can traverse from router to router.

But when I do ROUTER(subnet-A)------------ASA(subnet-B)------------ROUTER(subnet-A) no traffic flows. Is the ASA not operating in promiscuous mode or something similar that I'm not understanding? My plan was to configure the LAN router with a secondary address on it's WAN interface to allow me to still access the ASA, such as ROUTER(subnet-A)------------ASA(subnet-B)------------ROUTER(subnet-A + secondary address on subnet-B)

What am I missing here? is this even possible?