It is NOT a bug, it’s just not the data you want. The created_at field is correct, as has been said previously, it’s just it’s the date of when that subscription object was created in the database, not of when the current sub streak was created. That field was never intended to be used as a way to determine a persons current sub streak, so just because it’s not what you want doesn’t mean it’s inaccurate data.
For right now your options are quite limited, you can use chat badges or log sub messages in chat to get an idea of when a person started their streak but nothing accurate. If you want to know the specific timestamp a person started their streak you’ll have to hope it’s included in Helix (and there’s no confirmation of what will be in Helix).