Thank @coldrickjack for taking the time to help with this. I think the issue may be that I was attempting to use a private app, and private app webhook subscriptions can not be edited by api currently. There is a little disclaimer here: Accounts Dashboard | HubSpot
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Please note:
- Webhook support for private apps is currently in beta. Note that during this beta, webhook settings can only be edited in the in-app settings for your private app, and cannot currently be edited through the API."