I have reviewed the apt logs thoroughly, reviewing each update done since the last system start up, and verifying that it wouldn’t trigger such a restart. No culprit was found.
Anyway, I needed to get some feedback from community. So, thank you. The forum shows 34 views of this post (so far).
Ask your OS provider
I’m the system admin. It is my responsibility. I believe I’ve done the research. But I need to set up one more tool: An email notification when a reboot is needed, and luckily this has been discussed here recently, but needs to be tweaked to include LXCs and VMs.