There is an SB that relates to this, the pin mounting the door lock gas strut can bend over time reducing its effectiveness. A new hardened pin is available. Also the gas strut itself can become worn out. RSB42-125 refers.
The door catch has an "over centre" design and the gas strut should cause it to spring to the closed position.
If it doesn't spring closed then it can vibrate unlocked in flight.
Also as mentioned already in another post the red push pin can stick and hold the emergency catch in the open position. A quick squirt of WD40 will resolve it.
I make a point of testing the latch and the emergency catch every time I open the Cabin door.
How to loose your cabin door or I learned about flying from that...
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