People have already mentioned this, but...
This honestly doesn't matter. It's a performance at a music festival. Music festivals, even compared to a concert that is exclusively that group, have more "breathing space". They are, by nature, a lot more raw and a lot more driven by the energy the performers put out, the energy the crowd gives back and the hype that builds and sustains a set.
Yes, some of the clips of the LSF set are... well they aren't good. But none were show stoppers. The entire set went off and from almost everything I've seen and read of people who were THERE, the energy and good vibes remained high throughout. I've seen clips of people at barricade who were ecstatic getting to see LSF up close, screaming their lungs out.
The girls still put on a respectable, if not slightly flawed, show that gave the KEY audience what they wanted. Arm chair critics at home with more chance to quietly sit back and watch, rewatch and analyse performances without 10,000 other people around you absolutely vibing, screaming, dancing, and going the fuck off while you are there taking in the moment. Not to mention at festivals many of the audience are drunk, high, or otherwise influenced.
I don't honestly think these people indicated really cared as much about some of these vocal issues while in the moment as @armyblinkoncemydivetokkifan69 on twitter heard on repeat listens while searching for flaws:
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I've seen articles today where even seasoned veterans and legends like Blur and No Doubt struggled hard to get their crowds hyped up. But LSF managed to.
There CAN be bad festival performances. Where the artist leaves midset, where they completely fuck up, where there is a degree of antagonism between artist and audience or where the vibe just isn't there. That does NOT seem to be the case with the LSF performance at all though.
Even then, in my old punk/metal days I went to some festivals where the artists farked up big time. They were drunk, they could barely remember lines, they adlibbed nonsensical shit. But the energy. The damned well energy kept it all afloat and made for some memorable, amazing times because we weren't there for a technically perfect performance, we were there for a performance that was perfect for the time. (and yeah I know punk performances are different to kpop, but we are also talking festival acts)
And Knetz and their criticisms? From the concerts I've been to and seen video of in Korea, they definitely do concerts much much differently than American/Australian/Western festival goers. Where as festival goers mosh, party, vibe, and get merry, a lot of Korean concerts are much more passive. Much more "sit and watch intently (with the phone out)" affairs that much more demand a much more technically perfect show than one with high energy but more chaotic. Their tolerance for "bad" performance is much lower because of this. It's the same reason why clips of kpop artists from Korean concerts show them much more reserved, behaved and concentrating while the same artists performing in the US are being absolutely chaotic, twerking, fooling around etc. Different crowds, different energy, different expectations. See Blackpink, Twice, Dreamcatcher and even IVE for examples of that.
So yeah, this is a mountain out of a molehill.
In saying that, if this performance was at a solo LSF concert during a world tour, I'd be less accommodating. It's ONLY because it's a music festival performance that this is ok.