Grav CMS Cache + Mobile Detect Conflict

There seems to be an issue when using Grav's page caching.

When enabled the ```{{ isMobile() }}```  and ```{{ isTablet() }}``` vars only work intermittently on mobiles/tablets and even intermittently gives a false positive resulting in mobile style sheets etc being loaded for desktop browsers.

Same kind of issue with Drupal https://www.drupal.org/node/1889824
