Hello, everyone. I am not sure, but I think that issue may be related to the issue described in https://www.postgresql.org/message-id/CANtu0ojXmqjmEzp-%3DaJSxjsdE76iAsRgHBoK0QtYHimb_mEfsg%40mail.gmail.com
It looks like REINDEX CONCURRENTLY could interfere with ON CONFLICT UPDATE in some strange way. Best regards, Mikhail.