SUMMARY STEPS TO REPRODUCE 1. Open font previewer 2. Choose Noto CJK Japanese font OBSERVED RESULT Tofu appears (blocks instead of characters - even though Noto literally means 'no tofu'). Some other applications also show tofu when Japanese characters should be displayed EXPECTED RESULT No tofu appears SOFTWARE/OS VERSIONS Linux/KDE Plasma: Kubuntu 20.04 ADDITIONAL INFORMATION
> Some other applications also show tofu when Japanese characters should be displayed How about trying `sudo fc-cache -r` (and then re-login / reboot)? This will force re-generate all font cache files and rescan fonts.
This is an issue with the fontconfig provided by your distro. I would recommend reporting this to the Kubuntu people.