I think what you’re seeing is that the dash illumination (presumably including the touchscreen) dims automatically when the headlights are on. The Jazz’s auto-light function is notorious for switching the lights on and off as you drive into and out of bright sunshine, and the dash illumination is linked to that. That’s sensible because most people wouldn’t want a bright display inside if it’s dark outside. Page 178 (or thereabouts) of the manual suggests there is some control of this (not tried it myself):
“Instrument panel brightness varies, depending on whether the exterior lights are on or off. The instrument panel dims to reduce glare when they are on.
Pressing the <illumination control knob at top right of display> switches the display.
Turn the knob to the right until the brightness display is up to max. This cancels the reduced instrument panel brightness when the exterior lights are on.
The brightness can be set differently for when the exterior lights are on, and when they are off.”