Another issue is that GTK applications (Megapixels, Geary) are not showing most icons anymore. (Some icons do show up for some reason.) I have still not managed to find out why that is. I tried regenerating GTK icon caches in several directories, checking the gdk-pixbuf loaders (the SVG one is installed and found by gdk-pixbuf-query-loaders, the PNG one is apparently supposed to be builtin, I even tried reinstalling gdk-pixbuf2), even setting the GTK icon theme in ~/.gtk-3.0/settings.ini to Adwaita (normally, it is set to breeze with Adwaita as fallback), nothing helped.
UPDATE: Reinstalling adwaita-icon-theme (with sudo pacman -S adwaita-icon-theme) fixed it. I can only guess that the package got corrupted by the partial update that Discover did (see above).
With the above, clearly upgrade-related, issues fixed, I am actually quite happy with this update. The showstoppers that had kept me on the ancient snapshot are fixed: mobile data can be disabled without disabling the entire modem (there is one remaining issue that the state of the toggle does not persist on a reboot, which I am trying to sort out upstream with Devin Lin: Mobile Data always gets reenabled on reboot (#182) · Issues · Plasma / Plasma Mobile · GitLab), Spacebar now properly handles messages from non-numbers such as StadtWien and OE-testet (well, I have not yet verified that receiving them works, I will be able to confirm when I take my next Covid test, but displaying the archived ones definitely does – UPDATE: confirmed, receiving new messages from non-numbers also works fine), and whatever caused the unexplained power consumption and/or charging issue in the December update seems fixed, too (power consumption and charging looks normal so far).
I have tested it to work on my end. DP Alt Mode will always be an issue on rk3399, there is no real fix. All we can do is attempt to hack something together that might work.
I tested on my PBP, with the USB-C dock for the PinePhone on my 1080p TV. Only 1 orientation seems to work.