Hi Richard,
I believe that I am having a similar issue with the OP. I have been experimenting with the API but the results are not matching what is displayed on the adoption toolkit on our website. When I investigated further it seems the following animal IDs are not returning any results when searching individually (or even in other search endpoints where they should be returned). I compared two listings in the admin portal to ensure there wasn't something I missed.
I've tried both endpoints (test and PROD) as follows, and still get zero results for the following animalIDs (18029117, 18029120, 18038129, 18038132, 17927576, 18377960)
{API HOST}/v5/public/orgs/3189/animals/{ID}
{API HOST}/v5/public/animals/{ID}
Thanks,
Josh