How to fix Outlook calendar sync issues with iPhone and iPad after updating to 26.3.1?

First, Outlook Calendar does not sync with iCloud Calendar; it never has.

This is an understanding issue that you are attributing to the iOS update.

Please check which email account is set as your Default email Account for the Calendar on your device. Settings > App > Calendar > Default Calendar

All devices sync with an email client server that hosts your calendar. If you have an iCloud calendar, it will not sync with Google, Outlook, or any other Calendar, and vice versa. Settings > Calendar > Default Calendar [Choose what you want]. What you add (Event) to the iPhone Calendar is synced with the default calendar unless you have chosen a specific calendar at the time of event creation.

To which email account are the calendar events synced with your iPhone? This is not clear in your post

See the iPhone and Mac Calendar Pics below.

Pic1: Push and Fetch Settings

Pic 2: Default Calendar Settings: Which is your default calendar? Settings > Apps > Calendar > Default Calendar

Outlook Calendar Sync issues are not always the fault of iOS updates!  Many things can cause phone problems.  If you tell people you think it's an iOS update, it might make it harder to fix or find a solution for the real issue.

Remember, the right question often leads to the solution!

Was/Is your device Jailbroken? If that's the situation, please be informed that attributing the issues to iOS updates may not be accurate.

iOS updates aim to enhance user experience, boost security, and fix bugs rather than disrupt core features. The majority of the issues after updates might stem from underlying, unmentioned factors. Updates won't typically alter personal data but may refine settings or features. Backup before major updates for safety. Review Apple's update details and backup advice for a smooth process.

Kindly consider checking your settings and addressing any device issues before attributing the issue to external factors. This could be a pure coincidence that the iOS update affected the iPhone functions.

Before initiating a standard iOS update, it's advisable to create a backup. Have you completed this step? If yes, proceed with the restoration from the backup.

See what each update does: OS — iOS 26, About iOS 18 Updates, About iOS 17 Updates, About iOS 16 Updates, About iOS 15 Updates, About iOS 14 Updates, About iOS 13 Updates, About iOS 12 Updates, and so on...

There is no iPhone or iPad calendar. Instead, various email service providers offer calendar access to their accounts. These calendars do not talk to each other.