Cook bacon in a large pot over medium heat until browned and crisp, 7 to 8 minutes; transfer to a paper towel-lined plate with a slotted spoon. Reserve pot.  Add butter and leek to reserved pot. Cook, stirring occasionally, until just tender, 3 to 4 minutes. Add garlic and fennel. Cook, stirring, until fragrant, 1 minute. Add flour and cook, stirring, 1 minute. Slowly whisk in milk and then stock. Add potatoes and thyme. Season with salt and pepper. Bring to a boil, reduce heat, and simmer until potatoes are tender, 14 to 16 minutes. Add chicken and corn. Cook until soup is heated through, 1 to 2 minutes. Discard thyme. Serve topped with crispy bacon and chives.