@tut, no problem maybe I was a bit long winded. Yes, I think it is fine as is because it would make things too complex otherwise but @lubos might disagree.
I am in agreement with you in that I consider this a bug also. The exact same occurrence applies with negative tax codes. The programme allows for their usage, displays everything correctly in the view mode, but as per your initial post example then fails to transfer the view mode allocations to the actual accounts - tax not separated out.
As a workaround, as noted by your “tax codes work on all other balance sheet accounts”.
For your tax calculations to be automated (no manual calculation) you could use a BS > Liability > Clearing account Therefore, the clearing account would be used in place of the capital subaccount to create the tax transaction and then the clearing account balance would be reallocated via a journal back to the capital sub accounts.
Also, I disagree with others that the Tax module can’t / shouldn’t be used this way. Whilst it is predominately used for VAT/GST purposes their is nothing to suggest that it should be limited to that purpose only if its mechanics work absolutely correctly for other situations.
This situation, capital subaccounts tax codes, is similar to other areas within Manager where artificial restrictions are imposed within processes where accounting it self doesn’t have those limitations.