I think that this API endpoint (GET /api/v2/routing/queues/{queueId}/conversations) is deprecated.
It is not listed in the Routing API endpoints anymore.
If this request does not return your 20 conversations, I would suggest to open a ticket with Genesys Cloud Customer Care as we can't access and investigate customer data on the forum.
I think you would need to use an analytics query for conversation details - POST /api/v2/analytics/conversations/details/query - so it would require to specify an interval.
Note if some filters, like one leveraging queueId, are used in the query, the interval can be up to 31 days (max).
If you have more than 100 results(max page size is 100), you will need to page through the results (pageNumber = 2 then 3 ....).
Something like the following request body should allow to capture conversations which are currently sitting in queue ("desc" is used to get most recent conversations first):