Recurring appointments are not visible in OWA, but are visible in Outlook. (213612)

Situation: We did a migration in two phases. The first phase was migration of mail items and calendar items both with a data filter for items older than 31 august 2016. The second phase was a migration of mail items and calendar items both with a data filter for items newer than August 31 2016. Also contacts were migrated in this phase. What we see is that recurring appointments after the data of August 31 2016 are not visible in Outlook Web Access, but are visible in Outlook.

The recurring appointments in the series that are dated before August 31 2016 are correctly added to the calendar. The future recurring appointments after the data filter of August 31 2016 however are attached to the appointment series as attachments.

The Outlook client picks those up and displays those in the calendar on the right dates. OWA and possibly ActiveSync are not able to render those attachments and thus the appointments in the series after August 31 2016 are not shown.

The migration was from GroupWise to Exchange 2016. The issue looks like the one described in this article https://support.quest.com/migrator-for-groupwise/kb/186789, but the Outlook clients do not have a default storage location to PST, but instead to the mailbox. It looks like the filter (cutoff date) caused the issue in OWA When we did a remigration for a mailbox for the calendar with no data filter and the option RemigrateMode=0 in the gwmigapp.ini the recurring appointments were visible in OWA.

Question: What was causing the difference in view between the Outlook client and OWA and what are the steps we should take to fix this issue?

Do we have to do a remigration of the complete calendar with the option RemigrateMode=0