Melt butter in a large skillet over medium-high heat. Saute garlic until fragrant, about 1 minute. Add Cajun seasoning, salt, and pepper. Add shrimp and cook for 1 to 1 1/2 minutes. Turn shrimp over and cook, about 1 to 1 1/2 minute more. Transfer to a bowl and cover with aluminum foil to keep warm, leaving the butter in the skillet.
Place pasta in the boiling water; cook until tender yet firm to the bite, 8 to 10 minutes.
Add flour to the butter in the skillet; cook for 30 seconds. Add milk, cream, lemon juice, and soy sauce. Reduce heat to medium-low. Cook and stir until flavors blend, about 5 minutes. Add the shrimp and cook until well coated and opaque, about 5 minutes more.
Spoon the shrimp and sauce on top of the cooked pasta.