It seems this is a bug that has been fixed but we don't have the patch. Unfortunately, immediate patching of our cluster is not possible.
The 2 comments in the JIRA mention major compaction of involved regions as a workaround. Is this a legitimate workaround? While I've had some point success here, compacting a large table w/ this problem was not effective.
What else can we do to clean up the cluster? hbck identifies no issues w/ tables that have this error.
PS We moved our datacenter some weeks back. Post-move the cluster was not immediately stable. At least once we lost enough nodes that there weren't enough nodes to host all our regions and the cluster when down completely. After settling in, we experienced 2 weeks of stability before aforementioned problems began.