I’ve had that same issue and mine would make a distinct thud noise that was associated with a softer than normal pedal.Seems to me that should have made a noticeable noise before it finally let go.
Mine was so common I carried a wrench in the cab to tighten them when I heard the noise. I tried loctite and split type lock washers with no luck.
I bought some Nordlock washers and haven’t had any issues in about a year. My next step if that fails is drilling the bolt heads and using safety wire.