Flat alignment/rotation on slopes is incorrect in Software

Making an issue for this, even if it's commonly known by now.

In Software only, rotated flats on sloped planes are prone to inaccuracies, compared to OpenGL and what's shown in Zone Builder. ACZ1 notoriously needs manual flat alignment to make the sloped road flats at the start line up, at the cost of making it look incorrect in OpenGL (leading some people to believe OpenGL is incorrect).

Here's a test map: slopealigntest.wad

Some observations:

• Planes that are aligned by a linedef with an angle of 0 always look correct, while angles close to it seem to be relatively less misaligned? The worst misalignments tend to happen further away from the cardinal directions, in most cases.
• The positions of the affected sectors seems to have an impact on the alignment. Moving around the entire test map seems to change the severity of the misalignments, as well as which angles are the most misaligned.
• The steepness of the slope doesn't seem to affect the alignment, it's just an inconsistency between sloped planes and non-sloped planes.