That makes sense. Terrain mode is a simpler processing and is focus more on flatter triangles. What does the 3D model look like? It’s possible that it could stitch from the roof of the building or some point higher on the wall back to the ground just to fill in the gap.
The missing/uneven image spacing has been a problem that I have brought up and has been discussed at length. We ended up breaking the discussion into two particular problems. Missing Images and Uneven Image Spacing. Feel free to checkout the threads below. It just so happened in your case (as has happened to me in the past) that two missing shots were exactly where you needed them most. One of them appearing to be coming out of a corner which is a new problem that has arisen in the last month.