Him having an off night vocally I will always give a pass because a vocalist has a very difficult job, but forgetting lyrics and coming in late/early on verses when the songs are played to a click trick is kind of...not great.
I get what you are saying, but here's another perspective: Even in the non-prog world when you are talking about straightforward verse-chorus-verse-chorus-solo-chorus rock songs, the singer isn't singing the entire time. And during those times when the singer isn't singing, he is doing other stuff, whether it's getting a quick drink, interacting with the crowd, interacting with other band members, dealing with distractions, etc. During those times, it is super easy to momentarily get distracted from the song and lose your place. Just playing in covers bands and doing songs that I have heard a million times since I was a kid, and having sung along to them countless times, I still found myself a few times singing along, and then during one of those breaks, right before it is time to come back in, momentarily drawing a blank and having that panic moment for a split second of, "wait, what verse are we on again?" Most of the time, you recover before it is time to start singing. But a couple of times, I had to just mumble my way through part of a verse, or sing the wrong verse, until I found my place again. I think that problem is compounded when you have such complex pieces of music and complicated time signatures like DT. If you lose concentration for a split second, I imagine it is hard to recover.
And I get that these are well-paid professionals, and the argument could be made that it is their job to do everything in their power to learn to overcome distractions and lack of focus. In the abstract, I agree. I'm just saying, it happens, and I get why it happens. And, again, I think it is more likely to happen with a singer than other players because the singer stops singing throughout a song, whereas the instrumentalists usually do not, so there is less of a tendency for their level of focus to drop.