@Axel_Marciano @Mikolaj_Kowal @avinash4
We can confirm that @Phil_Seeman solution to create a new subscription is a workaround that enables you to retrieve all new events.
You can have multiple subscriptions on the same project (using parameter on target eg. /target/:projectId?v=2), but it might result in duplicate events when Asana has resolved the issue on their side. On our side we are working on handling duplicate events if the old 
subscription would start working again.
We have raised the same issue here: Webhooks: Scheduled Downtime in December - #8 by Jonas_O as we see that it’s likely to be related.