Don’t remember exactly how i solved this previously, please try the following:
Login > Projects > Your Project > Layout > Pro Editor > page header/include_head_system.tpl
look for the following, for me it was row 62-64
<meta name="HandheldFriendly" content="true">
<meta name="MobileOptimized" content="320">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
remove those three lines from the file, scroll down, click save and have a look if the mobile layout still changes.
