Files back-saved in 2019 cannot be opened in earlier versions?

I am currently the only user in my office that has upgraded to 2019. Because other users sometimes need access for revisions, I always backsave to 2017. 
They are still not able to open these files, and neither can I, when testing opening with 2017 on my machine. 
Note that I can still open all files through version 2019 on my machine.

"Error Reading *File Path Name*.cdr file."