Game is latest version (1.44). No third-party software. Checked existing 1.44 bug reports and upcoming fixes.
Simple issue that always comes up (unconditional). If you research a feature and finish it, there is a popup that appears, with a few options. One of the options is to apply the feature to specific cars. If you have enough cars that the window needs to be scrolled, using the mouse wheel in EITHER direction will only scroll up. You must click and drag the bar to go down. Essentially, the mouse wheel is useless when scrolling down in the aforementioned menu, because it always goes up.