shouldn't it say "sȳroti" instead? cf. http://wiki.dothraki.org/High_Valyrian_Adjectives#S.C8.B3z-type. btw, i suggest adding the special character input buttons to the comment section.
That table is postpositive only. At first I overlooked the following:
A small subset of type II adjectives, whose stems end in r, have an unexpected lun/sol.nom/acc.sing. ending in z. ... Aside from this anomaly, they are entirely regular type II adjectives, so, postpositively: [my emphasis]
Follow the general class II adjective table when sȳz is prepositive: http://wiki.dothraki.org/High_Valyrian_Adjectives#Prepositive_2
MadLatinist has confirmed this here: https://www.duolingo.com/comment/23730979
thanks a lot! indeed i did overlook this introduction completely. tbh, i didn't even get it after reading. if you hadn't put that emphasis ont the term "postpositively", i wouldn't have attached importance to it whatsoever. next step for me as a german native was to look up its meaning lol...