If zones are configured for a collection, it is totally possible and expected that we end up with an imbalance collection because there is only one shard in a specific zone, so that shard can't donate its chunks to another shard because it would violate zones policy.

If this happens, the balancer will continuously emit this log:

No available shards to take chunks for zone

we should double check which version are affected, for sure in 6.0+ we have this problem.