It could be me coming off of a decade of jam band touring but I like when they improvise things like that - even vocal melodies. It gives the performance uniqueness, it shows the artist is willing to play around with their art a bit. One of the things that turned me away from prog metal and metal in general in recent years is how static, clean, perfect, backing tracked, etc. everything about the show is. I like when there's mistakes, I like when the singer tries something weird and it does or doesn't work. It's performance art, and it doesn't have to be "good" 100% of the time to be interesting, and fun.
If I wanted a perfect recreation of the studio album, I would listen to the studio album, or watch some tiktok teen perfectly re-create it. Give me something interesting live! As someone else said, even if it's a "miss", in the moment you probably won't even notice anyway
EDIT: The most egregious example of this with bands for me has always been Between the Buried and Me. I love their studio stuff, and have seen them live a few times, but everytime it's like they're just there to perfectly recreate the album note-for-note (which is, to be clear, an impressive feat, just not necessarily what I am looking for live). Even worse, it feels like EVERY BTBAM tour is an "album anniversary" tour. When are we just gonna get a setlist with some surprises in it out of them? lol