IMO it's definitely the warmup and not a stamina issue. The performances have come after a few minutes of interviewing, which negates whatever warmup he had. Even Mike sounded better on the third encore livestream with Kiiara which suggests neither were warmed up for the interview shows--it can't be a stamina issue with mike because he doesn't sing much. More evidence that it's a warmup issue: they transposed Heavy even though Chester sang it well in original key both times he was warmed up (Livestream, James Corden)