Thanks for getting back to me, @arcangelini !
I managed to figure out what the issue is just now. We have a plugin called “AMPforWP” and they have an extension to make WooCommerce work in AMP.
So whenever someone bought using the AMP version of the site, it caused this issue.
Since it seems like the AMP WooCommerce extension is faulty, I’ll follow up with the plugin author for that but is there any alternative plugin that WC recommends for AMP?