Hi @HEATHERWEST-ROJ,
I found this unanswered question while doing a community cleanup.
As far as I know, the Canvas rich content editor has supported auto-save since 2020 (New Rich Content Editor: Auto Save (2020-03-21 Rel... - Instructure Community (canvaslms.com)). If it's a file upload submission and not a text editor one, I believe the answer would depend on whether your school/institution ahs enabled the assignment enhancements feature option. With the option enabled, uploads should save in a draft state until the students hit the submit button. If that feature is off, uploads would be lost if not submitted within a few minutes.
Hope this helps a bit!
-Chris