checkLogEntryRWC in jstests/sharding/read_write_concern_defaults_application.js selects the first global-log line that contains the command comment and the substring "writeConcern". That can match nested command.writeConcern on a Slow in-progress query, which does not yet have top-level attr.writeConcern.
The assertion then checks attr.writeConcern.w === "majority" and fails with expected undefined to equal "majority".

Parse candidate log lines as JSON and skip Slow in-progress query lines unless attr.writeConcern.w is already present.