I think I have exactly same problem, my "About This Mac" says MacBook Pro (15-inch, 2017), my model identifier is MacBookPro14,3.
The only place I get the information correct is in iCloud Web-Cleint Settings, everywhere else, like System Preferences - iCloud and on my Apple ID web page I have exactly what you are saying, just only "Mac" with no exact model identification and device icon is also wrong, it shows logo of macOS Sierra 10.12 not the proper device picture.
I'm also trying to find an answer to this, if I get it eventually I'll post here. Bye.
There is one quite stupid idea, that maybe device is so new, that it's not yet recognized by iCloud Device feature properly. Because for me also, all my older models are ok.