The bug

Minecarts on top of rails pushed with a slime block behind them bounce off it even if the rail is parallel to the slime block's surface. This causes them to move to the left or right depending on the location of the slime block. They always drift sideways in the same way.

Since the rail blocks their forward movement, minecarts should stay on top of the rails. They should only move forward if the rail is perpendicular to the slime block's surface (aka aligned with the movement direction).