UPDATE: I’m doing a full install of openSUSE Leap 15.4/Gnome in a VM right now. I’ll be looking for the behavior the OP has mentioned to see if this is something recent. Either way, I’ll report back here with whatever findings I have.
Ok, I just had an OS Updates package to install, so I tried to replicate the behavior you previously described. See below.
Now, you didn’t specify if anything else shows up on your Power Menu (or whatever it is that Gnome calls this) so I don’t know if this is part of the experience you have gotten, but what I can tell you is when I told it to Power Off, it did shut down.
Don’t look on this as a refutation of your experiences, and for a couple reasons. First off, this may not be the same kind of update scenario you were dealing with. Knowing exactly what kind of update(s) you had going on could be useful in replicating your situation. Second, though I doubt it, it’s possible that powering down a virtual machine is in some way different than powering down actual hardware.
Additionally — and this is definitely an area I’d like someone with much more experience in SLE/openSUSE than I have to review — are the updates you put on when you encounter this situation something that gets installed during the restart process itself, like how Windows handles certain core updates? Does such a process exist within Linux in general, and/or SUSE in particular? I make this point because I personally have yet to see any core OS updates (or any other updates) get applied like this in anything outside of either Windows or macOS.
GNOME Software generally implements offline updates only. PackageKit (which is backend that performs the actual installation) supports both online and offline install. KDE is using the same PackageKit and performs online updates (which resulted in killed session/interrupted updates more than once on Timbleweed). Leap 15.4 should default to gpk-update-viewere (“native” PackageKit client) which also does online installation.