The next time would be the first.
Undefeated Teams Heading into the CFP:
2014: 1 (FSU)
2015: 1 (Clemson)
2016: 1 (Alabama)
2017: none
2018: 3 (Alabama, Clemson, Notre Dame)
2019: 3 (Clemson, LSU, Ohio St)
2020: 2 (Alabama and Ohio St*)
Couple of notable teams left out:
2014 - Baylor and TCU got left out, both at 11-1 and Big XII One True Champion co-champs
2016 - Western Michigan was 13-0
2018 - Ohio St and Oklahoma were both 12-1 conference champs. Three undefeated teams, so someone was going to be left out. Only time a one loss P5 Champ was left out of the CFP.
2020 - Cincinnati, Coastal Carolina and San Jose St were all undefeated.