createDestination does not send challenge verification provided url in sandbox, I get a 409

[nyg84_74]

I've tried sending this request using both an API call and the API explorer.

Here is an example input:

{"status": "ENABLED",
"deliveryConfig": {
"endpoint": "https://tinyurl.com/597jnn35",
"verificationToken": "abbd1234-1234-aaaa-bbbb-12345678901"}}

When I execute this I get:
[409 Conflict]

{"errors": [
{"errorId": 195020,
"domain": "API_NOTIFICATION",
"category": "REQUEST",
"message": "Challenge verification failed for requested endpoint."}]} 

For the endpoint I've tried both my actual URL and a webhook.site url. And my logging shows that eBay never sends a request to the endpoint provided, the same goes for the web hook.sit url.
Is there anything I need to do to enable this in sandbox ? or is this just not working right now