I'm afraid it's like that by design - every time you touch the screen, the setting reverts to 'off'. I initially found it irritating too - and I'd still prefer it 'on' at all times by default when navigation is active, with the existing button able to toggle manually with any change persistant - however, you do get used to the crrent operation fairly quickly. I Just got used to the extra tap to turn it back on after any menu interaction.