When I attach an image from my camera roll to a task that is in a Teams Planner - the image is shrunk to 7kb 🥺 and making the super tiny. I have verified I have media setting to full size and if I upload the attachment from a mobile web browser to a personal task - it will attach correctly and full size.
See video showing work flow. Attached original image here as well.
Using iPhone 11 Pro
Teams app
