The in-game mouse sensitivity inversely correlates with the size of the game window. As the window gets smaller, the in-game sensitivity increases. A maximized/fullscreen window appears to have the correct sensitivity, assuming
MCPE-22135 is not affecting the sensitivity. If I had to take a guess, I would say that this is the result of the mouse moving at the scale of the desktop, rather than the game window.
MCPE-22135, the bug appears to only affect KB/M controls.