Possible VSD bug

I've been playing around with the VSD which is a great little tool for situational awareness. However, a couple of times now it's done something weird. It's hard to explain but during the final stages of an approach the runway and glide path symbols become 'fixed' in reference to the airplane symbol. In other words the airplane symbol 'pushes' the runway along in space at a fixed distance rather than moving any closer. The symbols disappear off the end of the display (it's nothing to do with the scale set). It doesn't happen all the time, and last time it happended I landed, took off on a short turn around without reloading the NGX or panel state or anything and it worked as it should. Bug or feature? Adam Turley. oh, before I forget the manual talks about a 'Rollout Arm' feature in the symbology for the HGS. Is this modeled?