I've RDS PostgreSQL DB with read-replica. Also, have one script which frequently checks the blocking query but I faced one issue yesterday where the blocking query was a startup(backend_type)process on read-replica. I've detailed logs enabled but I couldn't find anything suspicious there. What could be the reason of startup process is blocking other queries?
Your screenshot shows the WAL Recovery reader process, which is handling receiving and processing incoming WAL as part of physical replication. It is working properly, and you should exclude it from your monitoring script.
Wait states are described in the documentation for