Gadget Racers: distorted pseudo-3d rendering

[goshhhy]

OS: Ubuntu 18.04.3/x86_64, Manjaro/aarch64, others
MGBA Version: 0.8.0-6037-2368391f, others

On the game Gadget Racers (USA) for GBA, also known as Penny Racers (EU) and Choro Q Advance (JP), there is noticable distortion of the pseudo-3d rendering of roads in the game.

Under Higan, it looks like this, comparable to a real console:

Under MGBA, it looks like this:

Notice that on MGBA, the lines on the road appear curved to the left, and the finish line appears farther away. These screenshots were taken in the exact same starting position on the track.

This issue happens with both software and opengl rendering.

[RobertPeip]

Had the same problem in the FPGA core. The game is writing the Reference point shortly after HBlank is left(about 10% into drawing time). Other mode7 games seem to write while in Hblank(e.g. Mario Kart)

If the RefX/Y is latched by the time hblank is over, everything is fine.

[endrift]

This appears to be the transformation matrix not being latched, despite the affine translation being latched. This isn't fixable with a scanline renderer, if that's the case...