Peach Bowl. In 2018 they again went undefeated, but never even sniffed the CFP, only reaching #7 in the Final Rankings.
The AAC was really strong in 2017, but struggled a bit in 2018. 2018 UCF also suffered from having one of their two ACC games cancelled due to a hurricane. Maybe if those two conditions were different, UCF would've been ranked higher, but I don't think they jump from 7 to 4.