When there are events in personal calendar that were not created by the folder owner and he's a participant with NEED-ACTION status, and kolab_invitation_calendars feature is disabled (which is the default as for now) the events will not be displayed.

The reason is here https://git.kolab.org/diffusion/RPK/browse/roundcubemail-plugins-kolab-3.2/plugins/calendar/drivers/kolab/kolab_calendar.php;f73ccbaa8cd523ff62bc6bf538276513c2e15613$293

I think we should skip this partstat filter if invitation calendars are disabled.