Niantic has announced that Team GO Rocket is making Shadow Pokémon stronger in Pokémon GO as long as they remain Shadow Pokémon.

Team GO Rocket is making Shadow Pokémon stronger!

Trainers, We’ve received reports of developments regarding Shadow Pokémon. Whatever Team GO Rocket is doing seems to be making these poor Pokémon stronger as long as they remain Shadow Pokémon. Details are as follows.

• Shadow Pokémon now deal more damage with their Fast and Charged Attacks, but they also take more damage when attacked.
• Powering up a Shadow Pokémon and teaching a Shadow Pokémon an additional Charged Attack now cost less Stardust and Candy.
• Legendary Shadow Pokémon that you save from Giovanni are now more powerful.

During certain time periods, Shadow Pokémon might be able to forget the Charged Attack Frustration by learning a different Charged Attack with a Charged TM, though we haven’t figured out when that’ll be possible. Please stay tuned for more details. Despite our efforts, Team GO Rocket is growing stronger and acquiring even more resources. We’re not sure how they’re doing this, but please know that we’re investigating how to fight back against these ruffians. Let’s stay strong, Trainers!

Source: https://pokemongolive.com/en/post/shadowpokemonupdate/