Mastodon subscription/web push API never calls my endpoint

Question

I'm trying to get an AWS Lambda function to be invoked whenever a (bot) Mastodon account is mentioned. I've set up the lambda, API Gateway, and logging such that I can call my function manually:

curl -XPOST -i https://XXXXXX.execute-api.us-east-1.amazonaws.com/mention-hook -d '{"foo":"bar"}'

Each such (manual) call is logged in CloudWatch, and I can see the payload passed.

I've set up a mastodon account and added an application to it (with push permissions), and I'm able to call the api (by passing the application's access token in the header. I'm using Python to register my push hook:

r = requests.post(server + '/api/v1/push/subscription',
                   headers={'Authorization': 'Bearer {}'.format(token)},
                   data={'subscription[endpoint]': 'https://XXXXXX.execute-api.us-east-1.amazonaws.com/mention-hook',
                         'subscription[keys][p256dh]': pKey,
                         'subscription[keys][auth]': authKey,
                         'data[alerts][mention]': True,
                         'policy': 'all' })

and the response (as well as subsequent GETs to /api/v1/push/subscription) look good to me:

200
b'{"id":60703,"endpoint":"https://XXXXXX.execute-api.us-east-1.amazonaws.com/mention-hook","alerts":{"mention":true},"server_key":"[redacted]"}'

Mentioning this account does generate notifications in the web interface and within mobile clients (via their own proxies), but my lambda never gets called.

I've tried the different options for policy as well as all the different kinds of alerts. I've tried this with several different Mastodon servers (all of which successfully deliver pushes to the proxies for mobile clients). I've simply never seen any Mastodon server call my endpoint.