Thanks for the SQL dump, it helped me regenerate your example.
Technically the settings for the discount are partially at fault as the 'disc_order_by' column is meant to deal with this sort of issue, by defining an order priority that discounts are applied.
In you example sql dump, the order by values where both the same so the cart didn't know which to prioritise.
However, I've patched the library to help prevent this happening.
The patch is available via the usual Github repo.
