Sometimes, when I start the game the Nature’s Swiftness button is active. I can press it and use it. However, it never comes on again. That means through an entire dungeon. The button will press normally, but the spell is not active and pressing the button has no effect. People on the Druid forum say it just pops back on a minute after using (no matter what you do).
I just checked: no macros, keybinding and spell buttons are not from addons, disabling all addons doesn’t change the behavior, changed the key bound to another button. All have no impact. Even logging off and on the game, or exiting and reentering the game doesn’t activate it. Seems to be completely random.