1. Here's why

Flickering lighting is usually caused by the Global Illumination Effect when the sun and/or other objects are animated.

The Global Illumination Effect in Lumion analyses surfaces that are lit up by the sun or by artificial light sources. The effect then places probe lights that emit light in all directions near the lit surfaces to simulate reflected light. These probe lights are reshuffled when the sun and other objects move in a video.

1.1: To prevent the lighting from flickering in a Clip that uses the Global Illumination Effect, everything has to remain static, i.e. no moving sun, no animations and the Wind Effect should be set to a low value so that trees don't sway too much in the wind.