SUMMARY
Numeral System Mode radio buttons for "Hex", "Dec", "Oct", "Bin" have no effect on the entered number.

STEPS TO REPRODUCE
1. Select Settings->Numeral System Mode
2. Select Hex from the radio buttons
3. Enter a number, e.g. FF
4. Select Bin from the radio buttons

OBSERVED RESULT
Top display: FF

EXPECTED RESULT
Top display: 11111111

From Help->KCalc Handbook (https://docs.kde.org/stable5/en/kcalc/kcalc/numeral-system-mode.html):

(note that the KCalc handbook text  is from 2016 (!!), but it still reflects what I would expect)

"You select the numeral system with the radio buttons on the right. To insert a hexadecimal number for example, just do the following: Select Hex from the radio buttons. Then enter a number and note, that you can use the letters A to F for numbers larger than 9. If you would like to see this number in binary, just select Bin from the radio buttons."


SOFTWARE/OS VERSIONS

Linux/KDE Plasma: Gentoo Linux 2.15 (x86_64)
KDE Plasma Version: 6.1.4
KDE Frameworks Version: 6.5.0
Qt Version: 6.7.2