Those annoying gaps are in regular Amnesia too.

[Cranky Old Man]

If you play the original Amnesia again, and go to the Study level (with the dog autopsies), walk up to one of the convex corners in the beginning of the level, and look up into it. You'll be staring into space.
Convex corners where "corner_convex_short" and "welder02_convex" are involved, don't work seamlessly together. You'd think that you could cover it up with one of the pillars, but you'd be wrong, because the pillar is cut into by the pillars of the wall (unless you make it reeeally fat), and looks notably brown (while the walls are red) and the concave pillars don't reach high enough, and would look weird if streched. The best solution is probably to take a small plane with the "plane_black" texture, turn it up-side-down, and seal the gap.