Both tax modules "area" and "percent" define the TAX_SHIPPING setting in config.py.
- For the percent module this settings has the argument "requiresvalue='tax.modules.percent'"
- For the area module this argument is "requiresvalue='tax.modules.area'"
This results in the TAX_SHIPPING setting having two requiredvalues. In my case it the percent module seems to have precedende meaning the TAX_SHIPPING settings does not show up when the "area" tax module is selected. This makes it impossible to add taxes to shipping costs when using the area module.