The mount area was rusted and it wasn't working correctly.Husqvarna RZ4623 / Why did you replace throttle cable?
That's not the belt - that's the rusted throttle control arm.That belt looks like It has been through the war. Replace the belt and cleaner everything that has that coating of rust on it.
Make sure tensioner pulley on deck is moving freely and full length of travel. Make sure the belt tension is good. Possibly the deck engagement cable you installed is incorrect.That's not the belt - that's the rusted throttle control arm.
EDIT: Here are belt and pulley pics ...