This is a great question. Data from this website[0] suggests heat pumps leak at 6%/year vs. traditional condensing units which leak on average at 10%/year, so it seems they are still an improvement over traditional ACs. One technician's experience[1] points to the steel accumulator rusting being the primary cause of heat pump leakage. There was a big 2014 study in the UK[2] on heat pump leakage, but no causes were identified, due to poor data collection.