Thanks for the report! I tested both cards and they work correctly here: Restless Reef’s attack trigger mills 4 cards from the target player, and Garruk’s -3 makes each opponent discard two (and you draw for each one who didn’t discard two nonland cards). So it’s probably something specific to your game. Could you tell me:
How many players, and were there teams?
Right after it happens, open the “Log” (bottom left), copy it and paste it here. It shows exactly what the game did.
One thing to keep in mind: Restless Reef mills only the one player you target, and Garruk’s -3 hits opponents only, not teammates.