Luis I have SafeSearch and so forth disabled. However, I do use ProtonVPN, with Swizterland and assorted Nordic nations that my traffic is masked by. I have seen some content on JAST USA not come up while browsing, which is a adult gaming store.
Beyond that small possibility, I cannot think of a reason why the subscribstar.adult page isn't among the top five results. Strangely, an unrelated subscribstar.adult was listed after clicking the "more" button.
...OH! I figured it out. I specified "subscribstar.adult" in my search terms, because subscribstar has a regular and adult section, which the game is in the latter. However, it seems like that Kagi doesn't understand the .adult part. Just using "subscribstar" does list the site after several results.
With any luck, the Kagi team would be able to fix this discrepancy in search behavior.