I noticed this behavior. Is it new?
In question Q:
user A posts an answer, then
user B posts a comment under the answer of A, then
user A posts $n$ additional comments without an @B ping, then
user B receives inbox notification of $n$ comments in question Q.
I observed this as user B. The comments of A were not edited, so that the visible comment text is the original and one can see that it never contained an @B ping, or the username of B, or quoted text or shared keywords from the comments of B, that might have generated an automatic notice.
Does the software auto-notify when the comment discussion involves only two users, or only one user other than the answerer?