Items in my recurring invoice subscription weren't charged tax. How can I resolve this?

First, check in Seal Subscriptions > Settings > General Settings if you have the app set to keep the prices from the initial orders. 

If this is turned on and the price of the product in your subscription is different from the current price, then the app has to create a custom line item on this draft order. And because of that, Shopify took the default tax profile and applied it on this item. 

To resolve this, you can either:
a.) change the option in Seal Subscriptions > Settings > General Settings to always use the newest product prices (as this will bypass this)
b.) change the default tax settings in your shop, so that the products will have tax set on them by default.