There is a scenario where an OS-level crash can (sometimes) corrupt the document open in Scrivener’s editor. When this happens, the open document becomes unusable. I don’t know why an OS-level crash sometimes causes this and why it sometimes doesn’t, I’m just relating what I’ve read from the experiences of posters to this forum.
So what happened to you wasn’t that autosave didn’t work, it was that the open document was corrupted.
As @kewms says upthread, recovery of writing you did prior to the crash session would be from Scrivener’s zipped backups.
I work on a laptop with a battery, so a power outage scenario is an unlikely one for me, nor is my laptop prone to crashes. Still, I try to mitigate the risk of an OS-level crash by backing up the project to zip files early and often during my writing sessions. That is, I don’t wait until the end of my writing session to make a backup.
I recommend you do the same, unless your projects are so huge that backup time takes minutes.
See this post for more detailed advice and suggestions I gave to another poster who faced a very similar challenge as you.
Best,
Jim