Columns containing binary data must be mapped using an expression that uses the UNHEX function.
The underlying reason is that MySqlBulkCopy just uses the LOAD DATA command, which accepts string data, and I don't think there's a way to safely escape arbitrary binary data (e.g., that contains newlines.
This is definitely a usability problem for the API, though. At the (small) cost of reading the destination table schema, MySqlConnector could detect destination columns that are binary, and automatically set up the UNHEX expression. This would also be helpful for BIT columns, that need a custom expression, too.