Hmm… The problem doesn’t appear to be limited to KDE Plasma, either… 
I’m going to ping @philm because insofar I know, he’s the one maintaining the mhwd and manjaro-settings packages.
Upon closer inspection, I have found the following…
1 - The file ~/.xinitrc does contain an xset command to set the keyboard repeat rate and delay, and they are set to my preferences, but upon logging out of the GUI and logging back in again, Plasma reverts to a delay of 660 milliseconds and a repeat rate of ─ I think ─ 25 characters per second. These settings also appear to be overriding the Plasma-specific settings under…
System Settings → Hardware → Input Devices
… which are still set to my preferences, but which apparently do not take effect, just as my settings for the compose key and for always having the numeric keypad produce numbers even if NumLock is off are not having any effect anymore.
2 - Setting the keyboard repeat rate and delay to my preferences again via…
System Settings → Manjaro → Keyboard
… does work, but only for the running session. Once you log out and back in, they are reset again to the slower values as I mentioned higher up. The compose key settings and the numeric keypad settings however have no effect anymore, even though they are selected.
3 - I suspect that a system-wide setting overrides the user settings, and although I’m not a developer, I suspect that the update to systemd of a few weeks ago would be responsible for this.