It seems that the test JSON payload for webhook API differs in some ways from the actual payload. There was a post about this a couple of years ago (https://www.patreondevelopers.com/t/why-is-test-payload-data-structure-different-from-real-payloads/4357), but it seems that the data structure has changed since then too.
Can a developer please provide the current data structure of a webhook response (I’m particularly interested in members:pledge:create, members:pledge:update, and members:pledge:delete).
There should be a few threads with this info. Until the documentation is updated, you can look at those threads or pledge to your own creator account from different account from a $1 or lower tier and test the webhook response.
It could’ve been that I hadn’t updated my secret key, especially if the current format matches the link in the original post.
Edit: I’d just like to note that the payload in the link from the original post does seem to match an actual payload.
You can try creating a totally new, test webhook only for test purposes and use its key.