@Santhosh Balakrishnan Thank you. Indeed I am accessing the Planner from the tab in Teams. Felt sure I had uploaded the from shared drive using the tab but perhaps not. Are we saying this feature is simply not allowed using the tab? I'm surprised why not?
The reason for the design choice here @ColinFBMH, was that compared to the experience in the web it isn't as obvious where you'd want to put a document. Rather than re-invent the wheel it was thought that using the Teams UI to add a document and then the Planner UI to link could achieve the right result - and allow work on additional features rather than duplicating code. Sorry for the extra few clicks needed here.