It is possible for a Validation Function to run during cart interactions, but that wouldn’t really explain why it’s checking for an address that doesn’t exist yet.
As a first step, could you head to Settings > Checkout > Checkout rules in your admin and let us know if there are any validation rules enabled there? If so, try temporarily disabling them to see if that changes anything.
Also, if you could share your shop domain/URL, that would help us try to replicate this on our end and dig deeper into what’s happening. Happy to set up a DM on my end though if you’d prefer to keep that info private. Once we narrow down which function is causing this, we can look at why it suddenly started triggering for sure whether it’s an app update or something else.