Calendar events visible in Microsoft Graph API but missing in Outlook Web UI

Hi,

We are seeing inconsistent behavior between Microsoft Graph and Outlook when working with a specific calendar.

We have a dedicated calendar that belongs to the mailbox

MICROSXXXXFT_OUTLOOK_ACCOUNT_EMAIL (the calendar owner).

We create events in this calendar via Microsoft Graph API using

POST /users/{MICROSOFT_OUTLOOK_ACCOUNT_EMAIL}/calendars/{calendarId}/events

and read them via

GET /users/{MICROSOFT_OUTLOOK_ACCOUNT_EMAIL}/calendars/{calendarId}/calendarView?...
**
In the Microsoft Graph API response, the events are present and look correct.
example: **

{
  "id": "<PII:Moderator removed>QBGAAAAAAB_...",
  "subject": "test c3 1.2",
  "isAllDay": true,
  "start": {
    "dateTime": "2025-09-01T00:00:00.0000000",
    "timeZone": "UTC"
  },
  "end": {
    "dateTime": "2025-09-02T00:00:00.0000000",
    "timeZone": "UTC"
  },
  "webLink": "https://outlook.office365.com/owa/?itemid=...&path=/calendar/item"
}

We have two more similar all‑day events (test c3 1.1 on 2026‑02‑01 and test c3 1.3 on 2026‑09‑01) that are also returned by Graph.
This calendar is shared with another user with read permissions (can view full details).

Problem:

• In Outlook on the Web (OWA) / Outlook UI, opening exactly this calendar, only one of three events is visible on the corresponding dates.
• Using Microsoft Graph (/calendarView with the appropriate date range), all three events are consistently returned.
• We verified the date range and time zone in OWA; on the days 2025‑09‑01, 2026‑02‑01, and 2026‑09‑01 only one event (which on the 2026‑02‑01) is shown in the UI, while Graph returns three.

Moderator note: Moved from Outlook Web Outlook.com Calendar