Oh no, it's not feminine plural xD It's a special adjective category.
(ο/η) δημοφιλής (masc. and fem.), (το) δημοφιλές (neut.)
(οι) δημοφιλείς (masc. and fem.), (τα) δημοφιλή (neut.)
So, since το νησί is neuter singular, it's δημοφιλές.
There is nothing about this type in the tips and notes, but we plan on adding the info to the new tree, as well as make a seperate skill for adjective categories that are a bit more uncommon, like this one. ^.^