Good moring Rohde, Mary P,
To make it clear for you, I'd like to provide a fairly comprehensive explanation. I hope you don't mind spending some time going through it. Microsoft did not disable or remove the calendar flyout in Windows 11. The feature is very much present and intended to be a core part of the OS. What you're describing is a classic symptom of a policy or setting that is being forcibly reapplied by your organization's IT management system, which is why it "never lasts" after your IT department fixes it temporarily.
Your laptop is almost certainly enrolled in a domain or managed with a tool like Microsoft Intune or Group Policy. These systems allow your IT department to push standardized settings to all company devices to ensure security and compliance.
There are two primary policy settings that control the taskbar's functionality, one of them is Remove Notifications and Action Center. This is a legacy name, but in modern Windows, this policy can disable the entire system tray flyout, which includes the calendar.
When your IT department "re-enabled it," they were likely temporarily overriding or removing this policy from your specific machine. However, the central management system runs on a schedule, often every 90 minutes or upon reboot. When it checks in, it sees that your device is out of compliance with the defined rules and reapplies the policy, locking the feature down again.
In this case, I think you need to review the Applied Policies, look at the Resultant Set of Policy (RSOP) or Intune Device Configuration profile on your machine to see which specific setting is causing the lockdown. Then check Policy Precedence. It's possible a conflicting policy from a higher-level Organizational Unit (OU) is overwriting a local exception. The most sustainable solution is not to change the policy for everyone, but to create a new Active Directory or Intune security group (e.g., "Calendar-Flyout-Allowed"), place your user account in it, and configure the policy to not apply to that group.
I hope you find something useful here. It's also appreciated of you to accept the answer as a way to share your experience with the community. Thank you :)
Vivian