@MrsLPalazzi currently there are two different ways that you grade passback from Canvas to PowerSchool can be set up. One of these integrations passes the "Missing" label to PowerSchool, the other does not. Since I do not know what district you teach at (and I am sorry to hear you don't have access to Canvas Support), I can't determine which integration your currently using, but I suspect that it is the former. If this is the case only the score, due date, and submission date are transferred to PowerSchool and not the missing status. PowerSchool likely can deduce a late status from the submission date being later than the due date, but it appears that it can't deduce a missing status from the lack of a submission.
If your district is using the newer integration that is powered by the OneRoster API, the missing label should be synced to PowerSchool and you may want to reach out to your administration if it isn't (since you can't contact Canvas Support directly).