Since the previous version, thunderstorms are not visible at all. Neither clouds nor rain. You can tell by the random mob spawns and because you can sleep through the night even though the sun is up in the sky. That's why I am positive that it's a thunderstorm and not some other source of mob spawns
Vv is enabled. Pics are taken only seconds apart.
That's because you are in a Savanna biome. This is intentional behavior per MCPE-35743.