While a replica is in recovery, inbound update requests are buffered to the transaction log. After recovery has completed successfully, those buffered update requests are replayed. As of this writing, however, custom update chain post-processors are never invoked for buffered update requests. See SOLR-8030. To work around this problem until SOLR-8030 has been fixed, avoid specifying post-processors in custom update chains.
Using Custom Chains
update.chain Request Parameter