VaM doesn't do sweep physics, because that is super expensive. So instead of actually "moving" an object, VaM just checks for collision at each frame. At a low framerate a fast moving object could just happen to be just before hitting the floor and the next frame already through. Then no collision is registered and your object just appears to fall through the floor.It's a whole environment and I haven't figured out where the "hole" is.
Thanks for your help.