I am not sure if it is an issue with version 6.31.4 but when using HMPs I'd recommend setting PH_ITERATIONS to a value >0 to use the iterative physics solver which is more forgiving to contacts in dense environments. It might also help to set TERRAIN_CHUNK to a larger value.
This could happen if you have a highly tesselated terrain and/or object. A quick solution is to set the object to PH_BOX/PH_SPHERE instead which generates less contacts and avoids contact misses. Ideally your object should not be larger than 2x2 vertices of your terrain. So if terrain face size is 10x10 then your object should be smaller than 20x20.