Transactions Spec:

Drivers MUST override all other collection, database, or client readConcerns with the transaction’s readConcern. Drivers MUST add this readConcern to the first command in a transaction if and only if the readConcern is supplied and not the default.

However, if you set a readConcern on a collection and do an aggregate or count as the first operation of a transaction, the driver sends the collection's readConcern, not the transaction's readConcern. Instead, the readConcern that was configured with mongoc_session_opts_set_default_transaction_opts or mongoc_client_session_start_transaction, or no readConcern, should be used.