That setting is controlled by the browser's local storage. If local storage is cleared, then the preference would be removed and would need to be reset.
Additionally, if this were a shared workstation there would be the possibility that this preference could be overwritten by a second user on the machine.
If you find that neither of these scenarios are the cause of this preference being reset, then I would recommend bringing this issue up with PureCloud support as that wouldn't be expected behavior.