checkout auto convert price in original currency

Hello,
I’m writing to you as a last resort because I think I’ve tried all the plugin’s options.
My site offers subscriptions in several currencies and I use your plugin to display fixed prices.
Whether it’s automatic conversion or fixed prices, in either case I can choose a currency, see other pages in the site and go back to the rates: the currency chosen is saved as the one to use. But when I add the product (a variation to be exact) and arrive on the payment page, the price is displayed for a few seconds in the chosen currency, then the page refreshes and the original price in € euro is displayed.
I’ve also tried forcing currencies with countries and using a VPN to simulate a purchase in Japan or the USA, but in all cases, as soon as I get to the payment page, I switch from Yen or Dollar to Euro.
I don’t understand where this is coming from.
Thank you for your help.