After removing the Realistic Biomes addon(I’m presuming as well other VFX addons) and applying the vibrant visuals experiment, flashes appears for the first minute of gameplay when hovering over the sky.
How to replicate:
Create world with a VFX addon.
Exit and remove addon, select just remove without copying.
Experiments - activate vibrant visuals.
Reopen game and activate vibrant visuals.
This issue is being tracked at MCPE-218264.