Understanding the "replied" tag

[Xu Wang]

Dear all,

First, I am extremely excited to be a part of this list now. notmuch
has really helped me. Thank you go all individuals working to improve
it and to help others to know how to use it.

I would really like to know if a message has been replied to (e.g.
using a certain message id). It seems that all I need to do is check
for the "replied" tag. But often this tag is not there, even when
there has been a reply (I have confirmed this through the thread
display and checking the message that replied to the message to make
sure it indeed has header "replied-to:<MSG-ID>").

I have looked in mutt, and also I see many situations where there is
no 'r' flag, especially for emails sent from me.

The following returns true:
notmuch config get maildir.synchronize_flags

So at least both are giving same answer, but I'm not sure why not
saying "replied" thread is correct.

What is incorrect for my way of thinking about "replied"? Do I
misunderstand what it is supposed to do or am I not updating the flags
correctly?

Kind regards,

Xu