The achievement is supposed to trigger on the full pet page (https://subeta.net/pets.php). If this does not work let me know.

I did not realize you had already made a ticket. That is why I edited to let me know if that page does not work.

Well that is certainly strange! But glad to see it triggered. Could it be that your level was not 1000? I see you were close.

Definitely strange! Locking up as resolved!