I did have just one more thought... normally when this style of breaker trips, the lever drops down a lot lower than shown in your photo. If you haven't already tried it, pry down on that lever lightly to see if it mover further down. If it does, you can try again to push up on it firmly to see if it clicks into place.
I had one of these breakers do something similar once - it wasn't reset completely and would open up ever so slightly with vibration. My circuit breakers have the red trip button so in my case I could trip it so the lever came down completely.
Good luck.
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