Position "Fixed" moves when scrolling down the page on mobile

Question

Create a div with position:fixed;top:0. In the computer is normal, and does not move independently if I go down or raise the screen, but in the cell phone when I go down the screen it comes out, but if I go up again it appears fixed at the top again. Does anyone know how to solve this? below is the div:

<div class="menu">
  MENU
</div>

.menu{
 position:fixed;
 z-index:9;
 top:0;
 width:100%;
}