When a user’s post is automatically hidden due to multiple flags, they receive a PM from the site contact username, default system. However, they are not notified of this PM, nor does the normal green circle indicator appear on their avatar.
Steps to reproduce:
User A makes a bad post (or topic).
Users B, C, and D flag the post.
User A’s post is auto-hidden and they are sent a PM.
User A does not receive a notification about the PM/hidden post.
I’ve also noticed this when a moderator responds to a flag that I have made. Not every time, but sometimes it does not notify me that the moderator has responded.