I was able to isolate the issue on CodeSandbox: https://codesandbox.io/s/datagrid-hidden-horizontal-scrollbar-when-fullscreen-vjiyz?file=/index.js

Please open it on a widescreen (the footer gets smaller on mobile) and click on "Full screen". It won't display the horizontal scroll bar.

Inspecting the element, it looks like that euiDataGrid__virtualized height is bigger than it should. It isn't taking into account the pagination height here:

  if (isFullScreen) {
    finalHeight =
      window.innerHeight - toolbarHeight - headerRowHeight - footerRowHeight;
    finalWidth = window.innerWidth;
  }