Traditionally, updates on Linux systems are controlled by the user. You get an icon in the system tray that looks important; you click on it; it asks you if you want to install updates; you say “yes” or “no”; updates are applied, or not; when you next restart any applications that you have running that were updated, the new version is picked up. Data isn’t lost, because updates don’t restart the application. You can (and do) update the Linux kernel in this way, and your computer just stays up (usually running on the old version of the kernel until you next restart.) Mechanisms have been added over time to allow auto updates to take place for critical security patches (“unattended upgrades”) but these have typically to be opt in. And again, they don’t restart running applications.
Snap breaks this contract. The update channel for Snap is independent from the KDE updater (on Kubuntu), and seemingly the Gnome updater (on Ubuntu). If you consent to applying updates from the general system tray “updates needed” notification, Snap updates are not included; they’re not even listed in the pending notifications from the system tray. Snap updates only happen when the Snap updater is running, either if the application is not running or after the period of time required to force updates has expired. Snap updates happen without consent.
I would really, really suggest moving away from Ubuntu, and opting for the countless better alternatives instead, like Fedora (the best desktop, in my view), Linux Mint (a great desktop, but a bit more conservative than Fedora), any of the Arch derivatives (for bleeding edge and tons of fooling around with AUR), or Void (for those of us with taste). Or any, any of the others.
Ubuntu just does not seem to have its users’ best interests at heart, and Snap is the best example of that.