Some notifications still sent by system? (issue/bug)

I’ve changed the site contact username to a staff name.
However I saw that some users received “Congratulations on your trust level promotion!” message from system.
I want to avoid that as I feel like getting a message from “system” is impersonal.

Messages about getting promoted to trust level 1 have been sent correctly from the site contact username.

Any idea why?

Many thanks.