A note on automatic updates says that app updates should be installed immediately, but I didn’t find any confirmation for this. I left my PC for several days - seem to have no effect. Also no software changes were logged in dnf history.
Also I would be very thankful if you suggest me a log files I can watch for this mechanism traces.
You seem to be doing the automatic updates with discover (uses packagekit and not dnf) so the updates will never show in dnf history.
Updates to apps that are not being actively used will never show results, though you can use tools such as rpm or dnf to see what versions are installed.
The only time an app update may encounter problems is if the app is already open when it is updated and the user does something that the running version encounters a problem with updated libraries or runtimes. Restarting the app fixes these issues.
The system updates (kernels, drivers, etc.) are held in reserve and are downloaded but not actually updated until the user reboots. These are performed as offline updates by packagekit (discover).
I personally only use dnf for updates and do not allow packagekit to perform updates at all so have no information about how to reveal the actual updates it may have done.
pkcon get-transactions should be the PackageKit equivalent of dnf history list. It throws an error whenever I try it though. Otherwise journalctl --since=-3d -g packagekitd should show everything packagekit has been up to in the last 3 days.
This is probably about flatpaks, for that use flatpak history.
I guess that means nothing is being updated. By the way, you are not the only one, it also didn’t work for me when I tried it a while ago. In a rather misguided attempt to avoid slowing down the system while it is being used, Discover was set to update only after the computer is left idle for 15 minutes, which of course doesn’t always happen. A fix was made a little while ago reducing this time to 1 minute, I believe this will take effect in Plasma 6.3 which will come out in February.
I assume you still receive notifications… so for now manually clicking update is the way to go.