This is a forgotten report "https://forum.simutrans.com/index.php?msg=201850"
The graphic problem appears when this line is executed:
ASSERT_EQUAL(setslope(pl, coord3d(2, 3, -2), slope.all_down_slope), null)
Captura desde 2022-10-18 06-53-00.png
Captura desde 2023-04-01 20-26-21.png