Comment 7 for bug 1697243

If something else deletes a flow, neutron isn't going to log it because it doesn't know about it. It will only log when it deletes flows.

So if you don't see a log entry, there must be something else is probably deleting flows on the system. I suggest increasing the OVS logging levels and watching for the timestamp when all of the flows are removed from that bridge. Then check to see if neutron or nova did anything at that time.