Created attachment 138111 [details]
my /usr/share/X11/xkb/symbols/fr file, modified from original.

SUMMARY
[plasma-desktop] 5.21.4-1 
Preview of keyboard layout with tastenbrett 1.0 displays wrong character. 

STEPS TO REPRODUCE
1. in /usr/share/X11/xkb/symbols/fr, replace a key code by "dead_abovering" (see my /usr/share/X11/xkb/symbols/fr file, modified from original.)
    line 88: key <AE12> { [ equal, plus, braceright, dead_abovering ] };
2. in kde-plasma, launch /usr/bin/systemsettings5 → Input Devices → Keyboard → Layouts → select fr → variant Français (obsolète, variante) → press "preview" (that launches /usr/bin/tastenbrett --model asus_laptop --layout fr --variant latin9 --options -title Français \(obsolète, variante\))

OBSERVED RESULT
→ TASTENBRETT 1.0 SHOWS "Ú" (Unicode U+DA (hexa)) for key <AE12> { [ equal, plus, braceright, dead_abovering ] };
→ BUT USING KEYBOARD WITH THIS LAYOUT, YOU GET dead_abovering.

EXPECTED RESULT
Teastenbrett should show dead_abovering

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Linux xxx 5.11.16-arch1-1 #1 SMP PREEMPT Wed, 21 Apr 2021 17:22:13 +0000 x86_64 GNU/Linux
KDE Frameworks Version: plasmashell 5.21.4
Qt Version: qt5-wayland 5.15.2+kde+r19-1

Version of the program: tastenbrett 1.0
Version of the package: plasma-desktop 5.21.4-1

ADDITIONAL INFORMATION
Very silly/unimportant bug, but I fill it anyway.

if you use my file, the key is "=" ("=/+/}/°" if using nothing/shift/altgr/shift+altgr modifier key)
line 88: key <AE12> { [ equal, plus, braceright, dead_abovering ] };

attached files:
my /usr/share/X11/xkb/symbols/fr file, modified from original.