Just did a bunch of testing and wanted to document the results...

Wall coverings on partial walls are either aligned at the top or the bottom, based on which EPs are installed. All EPs align wall coverings at the top on foundation walls. For normal walls...

Wall coverings are aligned at the bottom in the following EPs:
Base Game
University
Nightlife
Open for Business
Apartment Life

Wall coverings are aligned at the top in the following EPs:
Pets
Seasons
Bon Voyage
FreeTime
Mansion and Garden

Therefore, for early EPs and AL, a diagonally-split wall can sometime appear to be a standard wall through the use of normal and foundation walls. However, the later EPs and especially M&G destroy this ability.

The pictures are of the exact same house, in the exact same neighborhood. The only difference is which EPs are used to edit the house. By using a normal wall (aligned bottom) on the lower level and a foundation wall (aligned top) on the upper level, the wall coverings hide the diagonal split. However, when both normal walls and foundation walls align at the top, there's no way to hide the diagonal split.