There is a setting in Admin panel:
add rel nofollow to user content: Add rel nofollow to all submitted user content, except for internal links (including parent domains). If you change this, you must rebake all posts with: “rake posts:rebake”
This setting is by default checked but all the links of parent domain is putting up with nofollow attribute.
Please use following steps to replicate:
1 Go to admin panel and search for setting “add rel nofollow to user content”, just to make sure it is checked
2 Create a new post and put link of root domain
3 Inserted link automatically append with rel=“nofollow” attribute despite of being a parent domain link