As specified in their announcement Stream Webhook Update the webhook is now a stream change webhook, not specifically a stream up/down webhook any more. It is not a bug, (although this webhook has been particularly buggy since the change for other reasons, the fact that you’re getting notifications on a game change is NOT a bug and is intended).
The easiest way to know if a notification is for a stream going live, or just a change of one of the stream properties, is to store the previous stream state so that you can compare that with the new notification and determine the change.