Hi @DBA ,
SQL Server 2014 SP2 GDR update is available through Windows Update. When you turn on automatic updating, this update will be downloaded and installed automatically. SQL 2014 SP2-GDR is an older version than SQL server 2014 SP2 CU18. It seems someone only apply the CU18 to the primary node of AG, and did not apply it to secondary. So the version is difference.
According to MS document SQL Server Instance Prerequisites for AG, each server instance must be running the same version of SQL Server to participate in an Always On Availability Group. Please apply SP3 for SQL 2014 and apply CU4 for SQL 2014 SP3. We can rolling upgrade SQL AG, please follow the steps from this document Upgrading Always On Availability Group Replica Instances.
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