I've had this issue for a while now. When trying to purchase a game from the store the input to enter the security code for the card is broken. Sometimes it cannot be clicked and targeted, in the event when it is targetable, entering any number value opens up the card "change" dropdown menu in the absolute top left corner of the window.

This is for the macOS native steam app, my workaround has been going to the steam website. I have a screenshot as well, could provide a screen recording too if it helps with debugging. Pretty serious problem for payments.