Auto sync keeps turning itself on

Any help appreciated!

Over the last few weeks the auto sync setting has been turning itself back on. I turn it off manually and after a few minutes it turns itself back on. I do use an automation app (tasker) however disabling it makes no difference.

I believe that for an app to change this setting it needs to have the 'write_settings' permission. Using F-Secure app permissions I've tried disabling a number of apps with this permission but have not found the culprit.

Any ideas?

Phone : HTC One (M7)
Android Version : 4.4.3 (Kit-kat)
HTC Sense : 6.0