Hi,
Thank you for the report. This apparently comes from a customization of your product page view rather than from HikaShop itself. Since version 6.4.0 the product page is no longer a single HTML form, and the default product page view tags the characteristic fields so the script can find the selected radio button. Your product page view was customized (overridden) before that change, so it does not have that tag, and when the characteristics are shown as radio buttons the script could not read the selected value and the displayed price stayed on the default variant. The correct variant was still added to the cart at the correct price; only the price shown on the page did not refresh.
Looking into it today, we have improved HikaShop so that variant selection now works even when the product page view has been customized this way. You can simply download HikaShop again from your account and reinstall it to get this improvement. Your data and your customizations are kept.