FYI: I ended up modifying /mod/assign/gradingtable.php in the class constructor from
parent::__construct('mod_assign_grading-' . $assignment->get_context()->id);to
parent::__construct('mod_assign_grading');This effectively undoes the 'view all submissions' modification affected by MDL-67702 so that the behaviour goes back to remembering the filter across different activities. Would love this to have a system toggle, but this is good enough for now.