Thank you for contacting Canvas Support. We're sorry to hear you've been experiencing issues with EdPuzzle grades in Canvas. There could be a number of reasons for the 0 overwrite - Do you have a late or missing policy to apply 0s to students that don't complete assignments? Possibly a default grade was set inadvertently. Do you have the assignment marked as "do not count towards final grade?" This can cause disconnect between external apps like EdPuzzle and Canvas. If the assignment in both Canvas and EdPuzzle appear to be set up correctly, then it may be best for you to contact Canvas Support directly.
If we can look at the specific course and assignments, it will help us pinpoint the cause of the issue and provide a solution. Thank you!