Hooktheory I Capter 4.1 "the one that got away" by katty pery
The 2nd measure iii note2 doesn't resolved and goes to note 4 which is strong and unstable
you said that unstable must resolved , but this song doesn't
Please tell me the reason
My 2 cents: the 2 note that sounds just before the iii chord in the 2nd measure can be thought of as providing the 7th for a iii7 chord. The 4 note then starts a new phrase after the rest.
Keep in mind, there are always exceptions to every "rule".