View #188 on GitHub to know more details
Querying the GRAPH API the default result contains only 10 events. When somebody has more than 10 events, according to no explicit $orderby is set in the query - some "random" events are missing from the calendar.
It can be solve by adding $top=100 query parameter in the outlook-service.js by editing the line:
const calendarUrl = '${apiBasePath}/calendar/calendarView?startDateTime={0}&endDateTime={1}&$top=100';
Thank you for your efforts Sire for developing this excellent extension!