MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

17.5.1.37 Replication and User Name Length

The maximum length of MySQL user names is 32 characters. Replication of user names longer than 16 characters to a slave earlier than MySQL 5.7 that supports only shorter user names will fail. However, this should occur only when replicating from a newer master to an older slave, which is not a recommended configuration.