Rmail does not mark buffer modified when unread message is shown

[Göktuğ Kayaalp]

I'm considering a switch to Rmail from Gnus.  I've converted my nnml
boxes to mboxes using a couple python scripts [1], and am fetching new
mail via mpop. My Rmail configuraiton is found below. When browsing the
Rmail buffer, after viewing unseen messages, the buffer is not marked
modified, and neither ‘s’ nor ‘C-x C-s’ save anythign.  When I look at
the Rmail header in the mbox itself, I see that the unseen attribute is
not changed, it is always U (understandably given I can't save the
buffer).  The only way to actually preserve the changes is to manually
toggle the buffer modified bit and hitting ‘s’.  What is going wrong?
BTW I've narrowed the problem down to the last two ifs in
‘rmail-set-attribute’, the then branch of the outer one is executed for
a reason that IDK.  And I've used Rmail with mpop before without
problems.

Rmail config:
gk-mail-home is a path to a directory.
(setf
 rmail-primary-inbox-list
 (list (getenv "MAIL")
       (expand-file-name "inbox" gk-mail-home))
 rmail-secondary-file-directory gk-mail-home
 rmail-file-name (expand-file-name "current" gk-mail-home))

[1] nnml2mbox.py and splitmbox.py here:
    https://github.com/cadadr/configuration/tree/master/bin


Thanks in advance!

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İ. Göktuğ Kayaalp       <https://www.gkayaalp.com/>
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