Prevent shipping tax from doubling when item qty goes from 1 to 2 in cart
Since taxes apply currently only at the line item level, the tax is correctly applying to the item+shipping cost of the line item.

The shipping max cost is making a order level change to the baseShipping after all line items have been adjusted.

We are looking into supporting order level taxes only, to avoid this, and will update this answer soon with a solution.

Thanks.

UPDATE: This has been fixed, see answer here: Craft Commerce: Calculating (Product + Shipping) + Tax?