Aw man. I've rewatched the Friday video several times and figured out what happened. Since there's a big slow-down in the bar before the credits, Williams subdivides the last beat of the measure (beat 4) as two eighth-notes. For reference, a normal conducting pattern in a 4-beat measure is down (1), in (2), out (3), up (4).
However, since Williams wants to split beat 4 into two eighth notes, he actually beats beat 4 down (and kind of low), and then for the upbeat of beat 4 (so, the eighth-note before the credits, aka the second half of beat 4), he beats up. Everyone else seems to catch what Williams is doing. But if the trumpet player for some reason isn't able to see Williams kind of gently beating beat 4 down low, then when he sees Williams beat upwards he thinks "Oh, that's beat 4" and plays his pickup note after that.
...When actually, Williams' upstroke is the "and" of beat 4, the second half of the beat. That's why the trumpet player's note is so short; he thinks Williams' upstroke is the entire beat 4 (rather than the latter half of beat 4 which it actually is), so he plays his eighth-note pickup to the credits twice as short as it should be. He can plainly tell where Williams is about to put his downbeat for the start of the credits, but since he mistakenly thinks beat 4 is so short (when actually, it's so long that Williams subdivides it!), he plays his pickup note short. Since he doesn't notice Williams' downward beat 4, he probably thinks beat 3 is actually the one being super-stretched (almost like a mini fermata). It's just a fundamental misunderstanding.
In other words, the guy doesn't seem to be catching Williams' actual beat 4. And since beat 4 is usually conducted as an upstroke - not down - the trumpet player thinks he's seeing beat 4 being conducted, when actually he's seeing the second half of beat 4 being conducted. Sorry if this is confusing; I hope someone else on the forum who is musically-literate understands the point I'm trying to make.
Go see for yourself in the video; can anyone else confirm they're seeing what I'm seeing? It's a bummer. The thing is, as a musician (and a brass player), I can see why the trumpet player would think the way he did. The issue could have been avoided if Williams conducted beat 4 as a partial upstroke (which would have made everyone plainly aware of where beat 4 was), and then re-beat up in the air to signal the second half of the beat. So basically, conduct beat 4 in the normal direction (upwards), but do two gestures upwards to subdivide the beat, so everybody knows what is what.
TLDR: Normally beat 4 is conducted as an upstroke, but Williams conducted beat 4 as a downstroke so he could do the second half of beat 4 as an upstroke. The trumpet player mistakenly thought the upstroke Williams did for the second half of beat 4, was actually the beginning of beat 4. That's why the trumpet player's last note before the credits is so late and short.
EDIT: Yep, he played it correctly on Thursday night; listen to how his note is longer and lines up with the rest of the orchestra. So, I'm 99% sure that's what happened. The other times, the guy got confused by what Williams was doing for beat 4 of that measure, and you can hear he sounds a little uncertain as to the placement of his note. Too bad; hopefully Thursday gets used for that spot in the commercial recording, but I won't hold my breath.