The mongo shell doesn't seem to honor the following statement from the documentation: "If the transaction-level read concern is unset, the transaction-level read concern defaults to the session-level read concern."

An operation is always executed with the session-level read concern (if set) while the eventual transaction read-level concern - set on the session when starting a new transaction - is completely ignored.

It should be adopted a solution similar to the one implemented by nodejs driver that is assigning a read-concern to an operation respecting the correct order (transaction RC || session RC || client RC).