Banned, suspended, disabled, or otherwise unavailable users still show up in Helix’s followers list (and should these users ever become active, they’ll keep who they are following). Kraken on the other hand which displays a user object in the results, rather than just an ID, will just not have any result at all for those users, and you can even find out exactly what position in the list of followers that these ‘holes’ occur.
Most user endpoints that try to retrieve such a user just returns an empty array, which is why you’re not getting any data back. For the ID’s that you’re having trouble with, try this endpoint: https://api.twitch.tv/kraken/channels/<Channel ID>
If they are indeed banned/suspended/disabled/whatever you’ll get the response {"error":"Unprocessable Entity","status":422,"message":"Channel ... is unavailable"} which will explain why you can’t get a username from the ID, as it’s impossible to retrieve that user object (at least for 3rd party devs as far as I’m aware) and why Helix will not include them in the array when attempting to look them up…