I am pretty sure that width: 70%; is causing the problem. % always depends on the container size and therefore such problems can happen. Try it with using Pixels instead.

I had implemented an auto-complete functionality for my search box, but autocomplete is moving for different screen sizes it is static for above 600px. I want that to be below my search bar.

Mobile view

Desktop View

CSS for auto complete

  .autocomplete-items {
  border: 1px solid #d4d4d4;
  border-bottom: none;
  border-top: none;
  z-index: 99;
  /*position the autocomplete items to be the same width as the container:*/

  top: 100%;
  left: 0;
  right: 0;
  position: relative;
  display: flex;
  align-items:center;
  flex-direction: column;
}

.autocomplete-items div {
  padding: 10px;
  box-shadow: 0 2px 2px 0 rgba(0,0,0,0.16), 0 0 0 1px rgba(0,0,0,0.08);
  box-sizing: border-box;
  cursor: pointer;
  background-color: #fff; 
  border-bottom: 1px solid #d4d4d4; 
  width: 70%;
  max-width: 630px;
  position: relative;
  right: 64px;
  text-align: left;

}

/*when hovering an item:*/
.autocomplete-items div:hover {
  background-color: #e9e9e9; 
}

Search bar CSS

.mainSection .searchContainer .searchBox {
    border: none;
    box-shadow: 0 2px 2px 0 rgba(0,0,0,0.16), 0 0 0 1px rgba(0,0,0,0.08);
    height: 44px;
    border-radius: 2px;
    outline: none;
    padding: 10px;
    box-sizing: border-box;
    font-size: 16px;
    width: 70%;
    max-width: 630px;
    color: #000;
}