Just because something is the way it is it doesn't mean it has to stay that way, right? That is my initial thinking with this but as I ponder the question a bit more I think there is an issue. If an assignment is configured with an external tool, some of these tools have their own late policy / missing policy and pass that on to Canvas. And as we see with H5P some tools do not have such features. So I am wondering how Canvas would know what features the external tool has and what reduction has or has not been applied by that tool before the grade is passed to Canvas. Also, what would trigger an entry of a zero? If it is merely a non submission as of the due date, then what happens if the student subsequently completes the work on the external tool. How is that grade passing over? And what if the instructor intentionally entered the zero. I think these are some of the considerations for us here.
But we don't have to think through this 100%. Perhaps there could be an assignment subtype or such a setting that instructors can opt in to use when configuring an assignment or via Grades. I for one would like a zero to be entered automatically for missing work. Then if students post late, changing the cell fill color to blue alerts me to a need to review the late submission. Going back to my initial sentence, why not ask for such a feature?
By the way, the late policy notes in the guide are rather extensive. Of interest, I am not seeing where they come out and say the late policy doesn't work with external tools. I am adding feedback to the guide.
https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-apply-a-Late-Submission-policy-in-the-G...