A few thoughts. If this is only happening on one of the teacher's classes, and all classes have the same settings, I'm tempted to lean to a possible tech issue and would suggest submitting a help ticket to Canvas Support (Help menu on far left of Canvas, then select the option to Report a Problem).
Second, you mentioned this is happening when you masquerade as a teacher. Any chance that something is up with the settings for the user roles in this course? This seems like a very minor possibility.
Third, run a test. Have the teacher removes all filters in the Gradebook. Then close out of Canvas. Reload and launch Speedgrader from the assignment page rather than the Gradebook. Does this still occur? Is the teacher ONLY launching the Speedgrader from the Gradebook? I'm wondering if the issue might be tied to the workflow being used.