When the leftmost side of a UiButton is directly adjacent to the horizontal center of the screen, and the mouse is moved left over the center of the screen in one frame, MouseLeave will not fire. MouseEnter will not fire on subsequent re-entry of the button.
If the left side of the button is to the right of the center, the bug will only occur if the mouse is moved left fast enough to cross the center within one frame. If the mouse is moved back to the right half of the screen without touching the button, the button will no longer be highlighted.
If the left side of the button is to the left of the screen’s center, the bug will not occur at all.
Repro place: ButtonBugRepro.rbxl (14.4 KB)