I've hit this issue a few times and it is a bug in the canvas system that can be replicated.
This only really occurs, if like me, you cannot use or choose not to use differentiated assignments.
Specifically if using the Turnitin LTI (not CPF), multiple differentiation does not work.
In the example we have a number of resubmissions that occur within a weighted group
The original assignment at the top is open to everyone to submit, the additional assignment slot is created if a student cannot meet the original dead line date or get a chance to resubmit after the first assignment does not pass.
When a tutor is marking the assignments it is likely that to indicate the original submission will be updated to 0% to show other markers this has been reviewed and marked at 0%
Two assignment groups both weighted at 50%, each assignment is marked out of 100.
Assignment 1 the student does not have any problems with this assignment, a grade is entered of 57% for that student.
Assignment 2 the student has some problems and has to submit twice.
Assignment 1 = 57%
Assignment 2 = 57%
The overall course grade should be 57%?
(0.57 x 0.5) = 0.285
+ (0.57 x 0.5) = 0.285
= 0.57 or 57%
However if 0% is entered into the first submission slot for Assignment 2 then then calculation changes and becomes this:
This is what seems to be happening
57 / 200 (the 200 is the total marks possible for that assignment group) = 0.285 then it applies the weighting even though a 0% grade was entered in one of these assignments.
(0.285 x 0.5) = 0.1425
+ (0.57 x 0.5) = 0.285
Not sure if this is a new problem but suggestions to address it would be welcome
I would not consider this a bug as the gradebook is calculating the score as expected. What is happening in your calculation for 42.75% is that the first assignment group is worth 200 points and your second assignment group is only 100 points. This is because the gradebook will only calculate grades for columns that have a grade entered in them. Since you didn't use the second submission in the second group, it's not being factored into the grade.
Assignment One is 57/200, 28.5%, with a weighted score of 14.25.
Assignment Two is 57/100, 57% with a weighted score of 28.5.
14.25 + 28.5 gives you the 42.75.
I would suggest that if students submit a second attempt, you simply don't enter a score for the first attempt.
I do understand what you are saying and this is the present workaround I suggest to Tutors, it may not necessarily be a bug, but it is poor programming logic to assume an entry in the grade box is a positive value.
It does make sense from a Marking perspective if the markers are in different parts of the world to use 0% as a value indicates the assignment scored 0, what does not make sense is the programming logic then including that assignment in the final weighted grade calculation.
It may not be a bug, but it is poor design.
I think what is missing in Canvas is a way for instructors to create assignments with multiple due dates. Instructure did release an update to assignments that allows instructors to reassign an assignment. Perhaps that might be a better workfow for your tutors.