Forwarding VS and Route Domains

i'm not surprised, i think your SNAT pool should be in RD0, if you don't use the pool mechanism the RD0 traffic will stay routed in RD0 going through is default gateway. Thus he will expect a snat in the RD.

When you use the pool method , you directly tell bigip too leak to RD1 thus you will use a snat in RD1.

What you could do is breaking strict isolation and add a route in rd0 for your destination pointing to a gateway in rd1.