If "Walk through windows" is assigned to Meta+Tab, invoking it would only switch to the last window and subsequent presses of Tab (without releasing Meta) would have no action. i.e. It would only switch once to the last window and then stop working.
Which keyboard layout are you using?
(In reply to Martin Flöser from comment #1) > Which keyboard layout are you using? I saw that I previously didn't set a layout (settings > input devices > keyboard > layout > check configure layouts; added en-US) on my computer and, on you comment, corrected it to en-US. It seems to have fixed it after a reboot. Thank you.
As explanation: most likely the meta key was not recognized as meta modifier. Thus KWin did not enter TabBox mode as that requires a modifier to be held.