Multi-currency re-bill invoice incorrect

When generating a new invoice to re-bill expenses made in multiple currencies, the total value is incorrect.

The invoice is generated in USD as per configured customer preference and shows $ signs. However the amounts shown are actually the GBP values corresponding to the selected items, with the USD items converted to GBP.

When changing the invoice currency to GBP using the drop-down, the $ signs change to £ signs but there is no change in the values.

See attached screenshots, combined into one file to avoid new-user limit.

Shot 1 - selecting two expense items, one in GBP and one in USD.

Shot 2 - initial invoice shown, in USD but amounts are actually GBP

Shot 3 - invoice after changing to GBP, with amounts unchanged