width in pixels ignoring max-width:100%

I have a problem between width and max-width working in an element with a grandparent flex positioned container.

When I use width in pixels with max-width: 100% in this element, if the width is bigger than its parent it just ignores the max-width:100%. But if for example I use width: 150% then the max-width:100% does its work.

body{
  width: 640px;
  height: 360px;
  position: relative;
}
body:before{
  content:"";
  width:100%;
  height:100%;
  box-sizing: border-box;
  position: absolute;
  border: 2px solid black;
}
body:after{
  content:"16:9 screen size ratio example";
  position: absolute;
  bottom: 0;
  right: 10px;
}
#flex-container{
  width: 100%;
  height: 100%;
  display: flex;
  flex-flow: row nowrap;
  background-color: #95a5a6;
}
#sidebar{
  min-width:150px;
  flex: 0 1 150px;
  background-color: #7f8c8d;
}
#main-content{
  width:100%;
  flex: 1 1 auto;
  background-color: white;
}
.page{
  width: 100%;
  padding: 16px;
  box-sizing: border-box;
}
.grid{
  width: 800px;
  max-width: 100%;
  height: 200px;
  background-color: red;
  color: white;
  padding: 16px;
  box-sizing: border-box;
}

<div id="flex-container">
  <div id="sidebar"><h2>Sidebar</h2></div>
  <div id="main-content">
    <div class="page">
      <h1>Content page</h1>
      <div class="grid">
        A grid with certain width defined in units and based on the device. Specified width should not exceed its parent width (max-width: 100%).
      </div>
    </div>
  </div>
</div>


<br><br><br>


Should work like this:
<br>
<div style="width:640px;height:360px; border: 2px solid black;background-color:white;">
  <div style="width:100%;height:100%;">
    640px width parent
    <div style="max-width:100%; width:800px; height:100px; background-color:green;color:white;">
      800px width with max-width:100%
    </div>
  </div>
</div>

Here is the JSFiddle.