Hi,
I tested in my Outlook.com account but can't reproduce your issue.
- Go to Calendar section.
- Add a new calendar:


- Select this calendar.


- Close the browser and reopen, the calendar was still selected.
In this situation, please try clearing your browser cache and see if there is any difference. It is also suggested to test with other browsers.
In addition, since here we mainly focus on general issues about Outlook desktop client but your question is more related to Outlook.com, so if your issue continues, it is suggested to post a new thread to the dedicated Answers community for outlook.com:
https://answers.microsoft.com/en-us/outlook_com
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thanks for your understanding
and hope your question will be resolved soon.
Regards,
Aidan Wang
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