Numpad is working as intended (as in following the indicator on my keyboard) with other software, e.g. xterm, browser.
Found this problem on Konsole, Kate, Systemsettings, Dolphin, KolourPaint, Spectacle, Okular and Libreoffice.
Did not find this problem on Kcalc.

When SDDM starts Plasma, the Numlock indicator(previously turned on manually) on my keyboard will automatically turn off.