As per https://github.com/openshift/openshift-docs/issues/19847#issuecomment-656030450 this issue maybe happen when upgrading the cluster. But from teh description of the bug does not have any information about upgrades. Can you please confirm?
There is an upgrade bug depicting similar behavior https://bugzilla.redhat.com/show_bug.cgi?id=1852618. I was not able to repro this ovs lost flows issue on OPEN PR https://github.com/openshift/sdn/pull/158. While its still not sure (from dev) whether this change could solve upgrade issue or not but it looks promising and can be verified for upgrade as well once merged.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.