Trainers, attention! Melmetal has been temporarily removed from the GO Battle League as Niantic continues to investigate an issue with its stat modifying Charged attacks. The entire issue has been shared on the Niantic Support website (Known Issues section):
Melmetal using a Charged Attack that increases or decreases stats causes the Charged Attack and Switch buttons to disappear for both Trainers.
- Issue description: After Melmetal uses a Charged Attack that applies an effect that increases or decreases stats, both Trainers will have their Charged Attack and Switch buttons disappear, and will be unable to take any actions.
- Issue status: Investigating. Melmetal has been removed from GO Battle League in the meantime.
Do note that this is a temporary issue and that Melmetal is slated to return into Pokémon GO Trainer Battles soon.