Paul Rowe

Thanks Dwight.  What you describe is what Adam and I were seeing as well.  It was driving me nutz that it worked 'for awhile.'   

It might be worth mentioning that Push topics are very legacy SFDC.  (You may know that already.). https://developer.salesforce.com/docs/atlas.en-us.api_streaming.meta/api_streaming/pushtopic_events_intro.htm

Important

PushTopic events is a legacy product. Salesforce no longer enhances PushTopic events with new features and provides limited support for this product. Instead of PushTopic events, consider using Change Data Capture events. To learn about Change Data Capture events, see the Change Data Capture Developer Guide and the Change Data Capture Basics Trailhead module.

From Configuring data sync from Salesforce to Zendesk

I think you need to add this:

To set streaming API channel permissions

1. On the Salesforce Setup page, select Administration > Users > Profile.
2. Select the user profile of the admin setting up the integration.
3. Click Object Settings > Streaming Channels, and under Object Permissions, select the Read checkbox.