Hi Rt13,
From the description, to narrow down the root cause of the issue, try the following steps to check if the related/older items sync properly:
1. Use the default Calendar without a third party calendar application to sync the items.
2. Re-sync the calendar item information about the user account on the third part calendar application.
3. Sync another user account calendar item information with the third party calendar.
Thanks,
Ida Qiu
From the description, to narrow down the root cause of the issue, try the following steps to check if the related/older items sync properly:
1. Use the default Calendar without a third party calendar application to sync the items.
2. Re-sync the calendar item information about the user account on the third part calendar application.
3. Sync another user account calendar item information with the third party calendar.
Thanks,
Ida Qiu