I've had something similar happen in my P09 a few times, I THINK.
When the hammer is back in SA, I will pull the trigger, it released the hammer, and as I let the trigger move forward, the hammer drops to half cock until I let go.
If I leave my finger still on the trigger after the hammer has released it will say in position sitting, if I pull on the trigger it will go back as if going into SA.
Hasn't happened enough for me to try and do anything about it, but it wounds like what you are saying.