Comment 1 for bug 696710

I think I found the reason for this. Autoptimize seems to combine all CSS code no matter what media they refer to. There is the line:

<style type="text/css" media="print">#wpadminbar { display:none; }</style>

In the code and Autoptimize puts this in the combined css for the screen display. The result is, that the admin bar is not displayed anymore.