[Dovecot] Seen flag getting lost

[Edgar Fuß]

We have two dovecot 1.2 instances sharing Maildirs on NFS. Indexes are local to the individual servers.
Occasionally (no idea how to trigger this), the Seen flag gets lost on some messages. I've verified that actually the ``S'' is missing from the filename.
I suspect something like server A caching the flags, server B setting Seen, and then server A flushing its cache for another change so overwriting what B changed.
Any ideas short of switching to 2.0?