Place the diced potatoes in a large pot and cover with water. Bring to a boil over high heat and cook until potatoes are tender, about 12-15 minutes.
Drain the potatoes and return them to the pot. Add butter, milk, salt, black pepper, garlic powder, and ground cumin. Mash until smooth and creamy.
Stir in the finely chopped onion and thawed peas until evenly combined. Set the filling aside to cool slightly.
Place one spring roll wrapper on a clean flat surface with one corner pointing towards you (diamond shape).
Spoon about 2 tablespoons of the mashed potato filling near the corner closest to you.
Fold the corner over the filling, then fold the left and right corners towards the center, and roll tightly towards the top corner, sealing the edge with a little water.
Repeat with the remaining wrappers and filling.
Preheat the air fryer to 375°F (190°C) for 3 minutes.
Brush each spring roll lightly with vegetable oil on all sides.
Place the spring rolls in a single layer in the air fryer basket, making sure they do not touch.
Air fry for 12-15 minutes, turning halfway through, until golden brown and crispy.
Remove from the air fryer and let cool for 2 minutes before serving.