try to set this values like this
TheMovement->NavAgentProps.bCanFly = true;
TheMovement->DefaultLandMovementMode = EMovementMode::MOVE_Flying;
TheMovement->DefaultWaterMovementMode = EMovementMode::MOVE_Flying;
TheMovement->MaxFlySpeed = 600.0f;
try to set this values like this
TheMovement->NavAgentProps.bCanFly = true;
TheMovement->DefaultLandMovementMode = EMovementMode::MOVE_Flying;
TheMovement->DefaultWaterMovementMode = EMovementMode::MOVE_Flying;
TheMovement->MaxFlySpeed = 600.0f;