customer overcharged for the value of the conversion in default currency

I’ve recently started seeing weird values in my customers’ check outs, and soon started getting complaints- my customers are being charged in my default currency for the value in their converted currency. They also dont see the site automatically in their currency as had happened til then. i’ve been refunding them when it happens, but this isnt a viable solution because it keeps happening.

Here’s an example- Customer in the US sees the shop in GBP by default (i’m in the UK). They add an item to cart valued at 80GBP, and in the cart this is converted into 99.36 USD- the price that should have been shown in the listing. They then pay 99.36 GBP at checkout.
I have some pretty furious people breathing down my neck, and rightly so.
i
If the customer adds multiple items to cart, this only happens for one of the items. This has happened for multiple items, from both older and just created listings.
This happens randomly for different products.
It has happened for these that I can tell ( most have sold several times without issue both before and after the error happened):
https://geekandartsy.com/product/kyber-crystal-pendant-with-light-blue/
https://geekandartsy.com/product/ahsoka-tano-rebel-symbol-necklace/
https://geekandartsy.com/product/secret-sunset-geode-dice-set/
https://geekandartsy.com/product/charmander-d20-die/
https://geekandartsy.com/product/star-trek-pride-pin-love-long-and-prosper/

Thank you so much for looking into this, i’d really appreciate it if you could let me know as soon as possible, i’m getting a bit desperate because my reputation is going down the drain fast.