Are you using Wayland on the Debian 9 ? I don't use Cinnamon, and was not using it before, so that is the big difference between yours and mine, the point is though, I had xorg as my display server , and when I upgraded it stayed the same.
I did not miss the fact that it was a upgrade, however, I see this now:
https://www.debian.org/News/2019/20190706In this release, GNOME defaults to using the Wayland display server instead of Xorg. Wayland has a simpler and more modern design, which has advantages for security. However, the Xorg display server is still installed by default and the default display manager allows users to choose Xorg as the display server for their next session.
You should read it all, but since you might not have time, and if you want to take my word for it, it just does not say about Cinnamon , it does show it is available, but says nothing about Wayland being the default for Cinnamon, like it does for Gnome. I would think that means it is not the default on Cinnamon.
When time allows I'll attempt to do the upgrade again and try to pay better attention to the details ----snip---
That would be best, my experience is that doing installations, or upgrades, it is important to take my time, be sure to have the time, and pay attention to what is going on, it is slow, and a bit tedious, but important, when I did my upgrade, there were some questions asked in the process, off hand I don't remember what they were, but at one point, I had left the room, expecting it to proceed, went to eat dinner , when I got back, expecting it to be finished , nooo, it had stopped and was patiently waiting for my response, if I had been there watching , it would have finished faster, because I would have responded, and it would have continued,... so any way just saying,... It is important to take your time on it.