Vat not added to net figure on invoice line

When I enter a net figure on sales invoice the vat is worked out correctly but the total figure is still the net figure. In the box below is the gross amount as it should be. My problem is this has only happened for the last couple of invoices. Previously it has always added the net figure & vat together in the total box. I have tried logging out and going back in but makes no difference. Any suggestions?

We’ve never shown gross amounts on the invoice line, always the quantity, item total and line total (NET). The gross figures are always displayed in the footer.

We haven’t made any changes to the invoice screen in general for around 3 months.