I’m speculating pretty wildly here but reading the Act literally, it seems that if the motion is presented in the form that the substitute member should be permitted for any meeting that the elected member doesn’t attend, then they are entitled to vote at subsequent meetings and therefore could count towards the quorum.
This all pretty arcane stuff and would only come up in a scheme where there was considerable discontent about the way the committee was running things. I also doubt very much if decisions made while the committee was technically unsound would be overturned or rescinded.