I found the issue. According to the Pipedream API documentation, the correct parameter name is app, not app_id. Looking at the search results:
From "List accounts" documentation:
app:
string
Description: The app slug or ID to filter accounts by.
Example: ""
But the SDK is using app_id and sending it as app_id in the query parameters. This is a bug in the Pipedream SDK! The SDK should be mapping app_id to app when making the HTTP request.