I think it's because of how the tags link to each other on ao3 (this is a guess I have no idea how the wrangling works) - if you search that ot3, it'll bring up fics that have part of that relationship tag e.g. Candy Morningstar/Lucifer Morningstar and even Candy Morningstar & Lucifer Morningstar. But the ot3 itself does appear to have 0 fics currently in any language, so you're good to go for both!!

Happy smashing!