Session are getting dropped post upgrade 15.x.x

I suggest you open a case with support since you are experiencing degraded performance. Some thoughts to consider as you work through the issue:

1. Did you read all the release notes and note the changes of behavior and known issues?
2. Did you test in a lab against production-like traffic?
3. Do you maintain default profile settings or customize them all to avoid default behavior changes?
4. Is your failure-over device still on the old version and can you fail back to check discrepancy?