There is currently a flash of undisplayed Pokémon when it Mega Evolves, as the sprite hasn't been loaded yet. Both its sprite and its cry should be preloaded before battle. Additionally, the same should be done for both different form(e)s of each Pokémon, as well as various filters, such as the shiny filter.
Preloading of normal sprites has been disabled for now too, due to an earlier bug. This needs to be addressed at the same time as the Mega Evolution sprites, etc.
There is currently a flash of undisplayed Pokémon when it Mega Evolves, as the sprite hasn't been loaded yet. Both its sprite and its cry should be preloaded before battle. Additionally, the same should be done for both different form(e)s of each Pokémon, as well as various filters, such as the shiny filter.