agent skips over emails

[Eric S Fraga <e.fraga@ucl.ac.uk>]

Hello,

One of my servers is an Outlook server (office365) which I access
through IMAP.  If I don't enable the agent for this server, everything
is fine (although slow).  If I enable the agent for this server, gnus
misses some emails.  If I look at the inbox with another tool
(e.g. outlook web access), I can see emails already marked as read that
gnus has no record of.  The .overview file does not have those emails
and the .agentview file has message numbers that include those emails,
indicating that it seems to have already seen them.

I can get gnus to find the missing emails by manually removing the
.agentview file but this means that gnus has to recreate this file when
I next open the group.

And then later another (seemingly random) email will be missed.

I am happy to try to debug this but I have no idea where to start.  I
have had this problem for years now.  I am using emacs snapshot.  The
problem is not reproducible per se: i.e. I never know if emails have
been missed and only some emails are missed.  The problem usually arises
once I get an error message from gnus that tells me that there are
duplicate overview lines.  But I don't know why these duplicates appear.

Basically, the agent system is great but seems rather fragile.

Any pointers would be welcome.

Thanks,
eric

PS - I should add that this problem *only* arises with outlook and
not with any other imap server or nntp or any other backend.
-- 
Eric S Fraga via Emacs 27.0.50 & org 9.1.13 on Debian buster/sid