[Users] Sent mail not visible even though present in sent folder, Linux version 3.17.8

[mnieuw at zap.a2000.nl]

We have the #mh directory on a shared partition, for multiple logins.
It happens occasionally that a sent mail is not visible to the users that
sent it, but is visible from another login.
The sent folder has properties (ls -lrt)
drwxrws---  2 root homeusers 278528 Mar 10 12:14 sent

the most recent messages looks like
-rw-rwS--- 1 eri  homeusers      686 Mar  7 15:50 14926
-rw-rwS--- 1 eri  homeusers     4051 Mar 10 11:40 14930

The last one is not visible to user "eri", but visible to other members of the group.
Claws' configuration is the same for all.
Re-indexing the #mh tree made no difference.
Going back and forth between various logins makes no difference, it remain invisible
to the original poster "eri".

What is going on here? Config problem? Bug? Workaround?

Mat Nieuwenhoven