Also see Breaking changes in 8.6.
Shard rebalancing may temporarily unbalance cluster
From 8.6.0 onwards the default shard rebalancing algorithm will compute the final desired balance and then make shard movements to reconcile the current state of the cluster with the desired state. However the order in which the shard movements take place may be skewed towards certain nodes, causing the cluster to become temporarily unbalanced while the reconciliation is ongoing. As always, once a node reaches a disk watermark it will not accept any additional shards, but this skew may result in nodes reaching their disk watermarks more often than expected in normal operation. Once the reconciliation process completes, the cluster will be balanced again.
To avoid this problem, upgrade to 8.8.0 or later.
When the ingest attachment processor is used, the interaction of Tika with log4j 2.18.0 and higher (introduced in Elasticsearch 8.4.0) results in excessive logging. This logging is so excessive that it can lead to cluster instability, to the point where the cluster is unusable and nodes must be restarted. (issue: #91964). This issue is fixed in Elasticsearch 8.7.0 (#93878)
To resolve the issue, upgrade to 8.7.0 or higher.