Compatibility matters only at the level of major/minor release level, not at the maintenance level. For example, a 6.3 search head is not compatible with a 6.4 search peer, because the 6.3 search head is at a lower minor release level than the 6.4 search peer. However, a 6.3.1 search head is compatible with a 6.3.3 search peer, despite the lower maintenance release level of the search head.
May this be due to a bundle replication issue. or Synchronize system clocks -
It is important that you synchronize the system clocks on all machines, virtual or physical, that are running Splunk Enterprise instances participating in distributed search. Specifically, this means your search heads and search peers. In the case of search head pooling or mounted bundles, this also includes the shared storage hardware.
Otherwise, various issues can arise, such as bundle replication failures, search failures, or premature expiration of search artifacts.