I was flying a practice GPS LPV approach on full autopilot. The autopilot status under the altitude tape showed both V and L guidance properly set and surrounded by a magenta box. At a point on final past the FAF, the box around the V:NAV went from magenta to red and I lost vertical guidance. I had never seen this before. The plane seemed to descend below the glide slope so I disengaged it and flew it manually.

I haven't seen anything in the manual about this red box though it seems obvious what it means and was wondering what should the A/P do when you lose any kind of guidance on a approach?