The issue exists in Magento 1 and still exists in Magento 2, when we use the Adjustment Refund or Fee taxes are not calculated.
I see in the documentation : "In the Adjustment Refund field, enter a value to be added to the total amount refunded as an additional refund that does not apply to any particular part of the order (shipping, items, or tax). The amount entered cannot raise the total refund higher than the paid amount."
So I understand that these adjustments dont apply to shipping, items or tax. But in the case we want to partially refund an items, we dont have choice to use the Adjustments fields and then the taxes are not recalculated.
I think a better way should be to allow to refund one or more products with a custom price (with checkbox as on admin new order form) and if the products are taxable then the refund will correctly recalculate the correct taxes.