If it helps, it’s not just flag PMs. See the link above.
I was in a PM thread where I got no notification for the OP or the first reply, then I made the second reply, then got a notification for the third.
Note that if you add people to a PM after the fact it will not work right since that person is (incorrectly) added with a state of tracking instead of watching. This is a known issue.
Oh I see, so the automatic flag handling responses, which post standard replies to the “flag as PM” topics based on the button the mod/admin clicks to deal with the flag, are not generating notifications. I agree, something for @zogstrip to look at.