I believe there is an error in the list of purchasable moves for Metagross (Mega), upon mega evolving Metagross loses a large amount of it's purchasable moves. Upon comparison no other Mega evolving Pokemon changes it's purchasable moves when mega evolving bar Lopunny and Lucario who have 1 attack swapped for another (so potentially bugged there as well but to a lesser degree).
Moves Metagross (Mega) loses from the purchasable moves list:
Aerial Ace
Brick Break
Facade
Flash Cannon
Gyro Ball
Ice Punch
Icy Wind
Psyshock
Round
Secret Power
Sludge Wave
Snore