mongoc_client_read_write_command_with_opts ignores the passed read preference. PHP uses the generic command helpers to send commands to servers directly. PHP does server selection itself, but still expects to pass a read preference so it is applied as a $readPreference to the outgoing command on the wire.

libmongoc does not provide a generic command helper that both respects read preference, and parses writeConcernErrror. That proved to be a difficulty in parsing writeConcernError on an aggregate with $out/$merge. See this thread.