I'm using the Canvas teacher app, and the icon doesn't show up for ANY submissions right now. It used to show up but doesn't anymore.
In addition, the icon used to show up even for Google doc submissions. Of course I couldn't use the Apple Pencil to write on those submissions, but it always showed up. Now when I go into Canvas, the icon never shows up anywhere. I also just updated my iOS, and nothing has changed.
With the release of iOS 15, there's a chance that you may see a hiccup that has yet to be documented. Can you double-check the version of your Canvas Teacher app? It should be Version 1.13.3.