If four of these assignments are worth a full 30% of the grade, then each one is worth 7.5%, so a zero drops her grade by 7.5 points out of 100 — which would certainly drop a 96 average to a B. An assignment worth that much is not what I would consider a minor “grade booster,” and definitely not something I would expect a student to lose track of. As many others have already pointed out, a dash in Blackboard or Canvas is not ambiguous: it means the assignment was never submitted. If the assignment was submitted but hadn’t been graded yet, that is clearly indicated.
I really don’t think it’s reasonable to ask a professor to “gift” a student with 7.5 percentage points she legitimately lost for not doing an assignment, when the only excuse is that she never noticed she hadn’t done it. Claiming that it was really the professor’s fault, because the professor didn’t immediately grade a nonexistent submission in order to remind her she wasn’t doing the required work, does not look good. The prof did not screw up. The way Blackboard and Canvas indicate unsubmitted assignments is not a software flaw that needs “fixing.” The student screwed up by failing to submit required assignments that unfortunately counted as a good chunk of the grade. Nothing to do really but learn the lesson, make sure it doesn’t happen again, and move on.