Preheat the air fryer to 380°F for 3–5 minutes so the basket is hot when the sausage goes in.
In a small bowl, whisk together Dijon, brown sugar, apple cider vinegar, garlic powder, smoked paprika, Worcestershire, and red pepper flakes until smooth and glossy.
For easier, cleaner cuts, place the kielbasa in the freezer for 8–10 minutes to firm slightly. Slice into 3/4-inch coins. Working one piece at a time, make 3–4 shallow crosswise cuts on the top of each coin without slicing through the base; set two butter knives or rulers alongside as rails to prevent cutting all the way through.
Spoon half of the glaze over the sliced coins and gently nudge some into the slits. Let stand for 5 minutes to lightly marinate while the air fryer finishes heating.
Arrange kielbasa in a single layer in the basket with a little space between pieces. Air fry for 6 minutes.
Flip the pieces, brush with the remaining glaze, and cook 3–5 minutes more, until edges are browned and the glaze is bubbling. For extra caramelization, raise to 400°F for the final 60 seconds.
Rest 2 minutes to allow the glaze to set, then serve hot with toothpicks.