So, 10.16.11.3 is some other router. Does it know how to reach 10.16.12.0? 10.16.12.0 knows how to reach rest of the world via default static route pointing to 10.16.11.3, but 10.16.11.3 also needs to know how to get back to 10.16.12.0. Your internal VLAN routing of the switch allows 10.16.11.1 to know how to reach 10.16.12.0 (via 10.16.12.1) but that does not mean 10.16.11.3 (external router) knows the same.
I assume 10.16.12.0 or 10.16.20.1 knows how to reach 10.16.11.3, but not the other way around. It is the replies that are lost.
YES?