Currently update_vbucket_map_history , works as below
When we get a newmap being set it checks if that map is present in the history, if so we do nothing(no reordering update the history as it was previously).
If the map isn't present we add it to the head of the list and delete the tail if the list is above 10.
Consider a situation where we have a large number of buckets say 9, with different vbucket maps, named bucket-1 to bucket-9. The vbucket_map_history lists is full at 10 past maps.
On rebalance, bucket-1 goes first it tries to set the vbucket_map_history sees that the map is already present in the history, but it is the last one on the list, we do nothing(no reordering update the history as it was previously) in this case.
The second bucket-2 rebalance runs and we discover that it has a new map we set the map in vbucket_map_history at the expense of bucket-1's latest vbucket map. The issue here is since the vbucket_map_history is not per bucket, it is per cluster, and we don't reorder according to latest map.
So after successfully rebalance we have lost potentially bucket-1's latest vbucket map.
Assume now we failover a node and add it back for delta-recovery, we cannot recover bucket-1 because we have lost that map.