We are experiencing this bizarre behavior, too. I checked the Microsoft Admin Center and do see that it is listed under service health as an advisory. Pasted the information from Microsoft below!
Users unable to see some channels within Microsoft Teams
TM288247, Microsoft Teams, Last updated: September 29, 2021 10:46 PM
Estimated start time: September 29, 2021 8:57 AM
Issue type
Advisory
Status
Investigating
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User impact
Users are unable to access some private or standard channels within Microsoft Teams.
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September 29, 2021 10:46 PM
Title: Users unable to see some channels within Microsoft Teams User Impact: Users are unable to access some private or standard channels within Microsoft Teams. More info: Users may also notice the following issues: - Users may be unable to view a subset of channels within affected Teams. - Users may be unable to view tabs within affected channels. - Some channel names may have been reverted to a previous state. As a temporary workaround, admins can still view the affected data using eDiscovery. More information on eDiscovery can be found here:
https://docs.microsoft.com/en-us/microsoftteams/ediscovery-investigation. Current status: Our investigation of visibility issues for the remaining affected channels continues. If your users are still unable to see some channels, please report this to support and reference this service health incident. Additionally, we’re examining the service for secondary effects that may be impacting customers and we’ll provide status updates as available. Scope of impact: Your organization is affected by this event, and the issue impacts an isolated subset of users that are accessing the affected channels. Start time: Wednesday, September 29, 2021, 8:57 AM (1:57 PM UTC) Next update by: Thursday, September 30, 2021, 5:00 PM (10:00 PM UTC)
September 29, 2021 6:53 PM
Title: Users unable to see some channels within Microsoft Teams User Impact: Users are unable to access some private or standard channels within Microsoft Teams. More info: Users may be unable to view a subset of channels within affected Teams. - Users may be unable to view tabs within affected channels. - Some channel names may have been reverted to a previous state. As a temporary workaround, admins can still view the affected data using eDiscovery. More information on eDiscovery can be found here:
https://docs.microsoft.com/en-us/microsoftteams/ediscovery-investigation. Current status: We've identified an issue affecting Teams channel visibility for users when attempting to access a subset of channels within the Teams client or web app. We've completed a service configuration change that has restored access for a majority of the identified channels. We're continuing our investigation to determine next steps to address the remaining channels and we'll provide a timeline for remediation when it becomes available. Scope of impact: Your organization is affected by this event, and the issue impacts an isolated subset of users that are accessing the affected channels. Start time: Wednesday, September 29, 2021, 8:57 AM (1:57 PM UTC) Next update by: Wednesday, September 29, 2021, 11:00 PM (9/30/2021, 4:00 AM UTC)