Microsoft Planner is able to work in offline mode when disconnected from the network. This note describes what the browser client can and cannot do when it’s not connected. The thing is that Microsoft hasn’t told anyone that Planner can work offline. No blog, no documentation, and no Office 365 notification. Isn’t that strange?
https://office365itpros.com/2020/09/04/planner-secret-offline-mode/
