If I enable do not disturb from the notification widget, a status appears e.g. "Until Today, 11:50", but the button does not appear checked.
Cannot reproduce. Are you using a non-default theme? Does it help if you clear your Plasma SVG cache with `rm -r ~/.cache/*plasma*`?
After a reboot, I can't reproduce it either.
Sounds like a cache bug; I suspect Bug 445516.
*** This bug has been marked as a duplicate of bug 445516 ***