Service Account for SQL Cluster
As per Microsoft, having same service account is the prerequisite :

All server instances that host an availability replica for the availability group must use the same SQL Server service account.
 The domain administrator needs to manually register a Service Principal Name (SPN) with Active Directory on the SQL Server service account for the virtual network name (VNN) of the availability group listener. If the SPN is registered on an account other than the SQL Server service account, authentication will fail.

https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/prereqs-restrictions-recommendations-always-on-availability