What is the reason for the diagonal bars on the bright display of Panasonic camera AG-HMC 153MC?

That's called zebra stripes, which is a way for camera circuits to evaluate highlights. It can be turned off on the fuselage or in the menu. This thing is for your reference, but the final product is not available. When the zebra function is turned on, if there are no diagonal lines on the screen, you should try to open the aperture or improve the light in the shooting environment. If there are too many highlights in the shooting environment and theme, it will usually be exposed and affect your final product.