Log in to iCloud

When you log in on a device that has not been logged in before, Apple will send a verification code to all your other devices that have been logged in before and are "trusted". Ergo, if your Mac is receiving a code it is trusted and should not need for you to enter a code since it is already known as being trusted. Get a verification code and sign in with two-factor authentication - Apple Support

You might want to ask Apple about this.

Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support

If you are getting the verification on the iMac that means you've already set it up as a trusted device at some point. Apple ID's/ iCloud accounts are two factor authentication, which means you need a trusted device and the password, or the trusted phone number and the password etc... two factor.

Two-factor authentication for Apple ID - Apple Support