In order to find the root cause of the issue more information is needed. What I can tell you for sure is that the code version that you are running is very old and is vulnerable so several problems.
We already have 8.20, so it would important to consider an upgrade in the future.
Perhaps you can attach the showtech of both CSS and I can take a look to see if something is wrong, also please clarify if when failing over back to the Primary the NATing started working again.
Hope it helps!!