This non-return air valve in the bottom of the air piston can stick open or become dicky when old or if it gets crap in it.
It can cause intermittent diving of the front suspension if faulty. Basically it holds pressure when pushed one way, and allows air to pass pushed the other.
If your Lefty occasionally dives and uses up alot more travel than is expected, this may be the culprit. Caused me grief today and was Wednesday’s catalyst.