I created a client with admin account then 1 website with 1 user I assigned the user to admin for the site. I set max domains to 1 so the user could not create second domain (actually that would work forreseller correct me if I'm wrog but the logic is the same. If I neet a user to have 1 domain I write 1)
Thats not a bug, its totally correct.
The first domain is the website itself, you entered it in the site settings. You can not add a second domain as you limited the number of domains to 1.