Hi @MsZ1,
While it may not be universally true, the way I've seen the external tools we have for assignments behave is that the external tool will work and will let the student go through whatever their motions for a submission are, and it will generally appear to work, but the canvas side will not accept the submission/score transfer from the tool once the submission limit has been hit. From the tech side, I understand exactly why the workflow is what it is, but it can be confusing for students because they don't really get locked out of making more submissions. There are a few LTI tools that have their own submission limiting system too, which will usually work better but is setup in the tool itself. My general advice to our faculty is to allow unlimited submissions on the canvas side when using an external tool assignment.
Hope this helps a bit!
-Chris
