Hello Newtoship,
There is no specific reason for that, and the ship shouldn't indeed turn. Nevertheless, the ship is only marginally course stable when making way astern, and depending on the height of the waves, some steering would be needed to stay on course. With the latest version, full astern on a flat sea, she will keep her heading up to 2 minutes without any steering at all, and I don't remember if any change to her stability would have been needed or asked in the past.
Regards,
Luc