Negative amounts are discarded when editing purchase invoice

I am currently running Version 14.6.5 and this seems to be a new bug which has not happened previously.
For Import Vat purposes I record the Import value amount with 15% VAT and add a new line which reverses the import value to just get the VAT amount. However, when I edit the purchase invoice, the negative amount is automatically set to 0. This is quite annoying, since the negative amount needs to be re-entered and special caution is required to not mess up the amounts.

When I edit the invoice:

Also, shouldn’t the tax column show when viewing the invoice, since some rows have VAT and some don’t? This is not the case. The VAT column is not shown:

Can you test the latest version 14.6.10? The reason for some of these regression bugs is addition of multi-currency. A lot of code has been rewritten to support it so some bugs leaked in. This should be fixed in the latest version though.