If it was working before replacement, it will be the screen assembly. You can rarely get one with a non-working camera, especially if it is not an original Apple part (for example, bought off eBay) and they have used a non-original camera and flex. The first 2015 15" assembly I bought when they were available had this issue, so it is likely this.
If it was not working before replacement, it will be an issue related to the logic board. Measure pins 2, 3, 4, 5, 6, 10, 11 and 12 of the front camera connector on the logic board (going from bottom to top) if it was not working before screen replacement, and let me know the values.