Manual Tax line created for zero on PO match and cannot be updated

We have a simple tax structure and want to turn off automatic tax calculation in 12.1.3.  We created a new configuration with the allow tax applicability turned off on standard invoices and PO's.  When we enter non-PO invoices everything seems to work ok.  We can enter tax information at the header level and the distributions looks fine.  However, when we match to a PO, the tax line shows as zero and if you look at the distributions it does a non-recoverable line for the correct tax amount and a tax rate variance for the whole amount as a negative.  We