The reason "Azantys" doesn't become "Azanti" similar to how "Vala" becomes "Vali" is because they are in different gender classes. If you notice, you'll see that apart from "Hontes" and "Azantys" all the other nouns we've learnt have ended in "a", like "Vala", "Taoba", "Kepa", "Abra", etc. This is because all those nouns are in the "Lunar Class" and the other ones are in the "Solar Class", and because of this, they conjugate differently.
Because you see who is acting/being by looking at how the VERB is conjugated. Iksat means "you are" (plural). Azantyssy simply means "knights" (plural). So you could say "Azantyssy iksat" but it would mean 'You all are knights' instead of 'They are knights'. I'm pretty sure but I'm not 100%. This is how languages like spanish are built so since this follows a similar type of pattern I suppose this is how it works in High Valyrian too.