Mediawiki 1.31: The message that CookieWarning produces does not seem to go through the MobileFrontend extension. Rather, the cookie message that appears on the mobile phone seems to be "below" (and partly overlapped by) the MobileFrontend-formatted text. It also does not appear to have the same width as the screen.
To see this bug in action, go to http://strucbio.biologie.uni-konstanz.de/ccp4wiki/index.php/Special:Version?useformat=mobile and inspect the top of the page - it's corrupted. It should look similar to the desktop version, http://strucbio.biologie.uni-konstanz.de/ccp4wiki/index.php/Special:Version (you need a desktop browser).
I have no idea if this is CookieWarning's fault, or MobileFrontend's.