No email notification when replying to a line comment on a diff.

There are two users A, B. Both use the "Participating" Email setting.

Step 1: User A commits and pushes his changes. User B adds a line comment on a diff, e.g. "Hello"
Expected behavior: User A receives email
Noticed behavior: User A receives email (OK)

Step 2: User A replies to the comment of B, on the same line of the diff, e.g. "Hello Back"
Expected behavior: User B receives email
Noticed behavior: No email is sent

The email to user B is sent if notification setting is changed from "Participating" to "Watch". Shouldn't the user also get a mail notification with setting "Participating".

GitLab Version 8.8.2-ee