1) update to the new version of the app 6.9.4
2) Go into iPad settings and scroll down to the "student" app (this is canvas student). Enable cross-website tracking.
Or you could stick with the web interface, but probably good to have a working app as well.
I have this issue as well. Students with school issued iPads were able to take a quiz through lockdown browser and view images in questions (png files). 2 students that had personal iPads were successful as well. Allow them had iOS 13. All students with personal iPads running iOS 14 could not see the images.
This is after having students delete the student app.