We expect the parallel shell to begin a w: majority write on Node 0, while Node 0 is primary and replication is blocked. Then Node 0 steps down and the parallel shell expects to receive PrimarySteppedDown.
In fact, the parallel shell took a long time to begin. It finally connected after Node 0 had already stepped down. When it began its w: majority write it received
This is not a "write error" but a generic command failure, so when the shell calls assert.writeErrorWithCode it logs unhelpfully: