Nick here from Shopify.
I made a little video showing instances of your issue, it's a slightly odd one:
So it looks to be occurring with certain variant combinations and not others, pointing to a there issue. Your theme support should be able to fix this for you. I see you are using a stock Shopify theme, however this may have been heavily customised, so let me know who and where from you got your theme, we should be able to put you in contact :)
Hope this helps!