ive finally figured out when the scrollwheel stops working with the hotbar: its with things that "eat" your scrollwheel inputs
choosing an action on a focused item takes scrollwheel priority over your hotbar, and when you get off the jeep the game seems to still think youre "focusing" on it, so your scrollwheel inputs get eaten by that. its reset by hovering over another prop. ive found its not just the jeep that does it though, sitting in chairs and inconsistently terminals will do it too