in my experience the chase count start from 0 if there is someone that evade, lets use jiraya toad chant combo chase, he attack 4 people, 1 evade, so the previous buildup combo reset to 0, and begin to increase with only the ninja it hit (so 5 combo x 3 unit that is hit result in 15 combo).

if the next chase require 20 combo, the chase stop since you only have 15 combo
if the next chase require 10 combo, the chase resume as usual since you have more than enough combo to continue

the only exeption to this i experience is if the "selected unit" of that chase is the one that evade. in that case, the chase stop even if the next chase only require 10 combo

in jiraya toad chant case you can see wether any other unit (beside the one that evade) get repulsed or not. if there is, then the "selected unit" of that chase is the one that get repulsed, if noone get repulsed, the unit that evade is the "selected unit"