SNAT flows not added when the first subnet is added to the external n/w after router g/w set.

SNAT flows not added when the first subnet is added to the external n/w after router g/w set.

Steps to reproduce
1)Create an external n/w.
2)Create a router and do router g/w set.
3)Add an external subnet to the n/w

Expected Output:

SNAT flows should be programmed.

Actual Output:

SNAT flows are not programmed.