Oh, I am really sorry. My coder sent the answer in the box chat, but I missed it.
Thanks for reminding me here.
It is because of this plugin: https://prnt.sc/mgUjk-f-n9EX
He guessed that you might use this plugin to customize the mini-cart or anything, so it led to the problem that the item in the cart was lost.
So pls try to deactivate the plugin and check your site again.
Thank you so much!