DO NOT upgrade a TiDB cluster when a DDL statement is being executed in the cluster (usually for the time-consuming DDL statements such as ADD INDEX and the column type changes).
Before the upgrade, it is recommended to use the ADMIN SHOW DDL command to check whether the TiDB cluster has an ongoing DDL job. If the cluster has a DDL job, to upgrade the cluster, wait until the DDL execution is finished or use the ADMIN CANCEL DDL command to cancel the DDL job before you upgrade the cluster.
In addition, during the cluster upgrade, DO NOT execute any DDL statement. Otherwise, the issue of undefined behavior might occur.