Special thanks to Vincent Roscioli for the first answer, which was totally correct. Sulfuric Vortex has a “symmetrical” trigger that can be usually detrimental or not depending on whose turn it is. Arnie missed the trigger on two different turns. The first time on Newt’s turn, the trigger was not usually detrimental for Arnie, so Newt isn’t required to point it out, and the judge should not intervene. On Arnie’s own turn, the trigger becomes usually detrimental, and the judge is obligated to step in to issue a Warning for GPE: Missed Trigger once it becomes clear that the trigger has been missed (in this case, it’s when Arnie draws a card).

As Vincent correctly noted, the trigger doesn’t immediately go on the stack; we ask Newt whether she wants to put the trigger on the stack (which will most likely be “yes,” but we still ask!).

Thanks to Jack Hesse for the reminder about how a player is never responsible for pointing out missed triggers an opponent controls, so Failure to Maintain Game State is not applicable here.

And special props to everyone for successfully ignoring the Sylvan Library, since it didn’t present any problems in this situation. Part of the skill of judging is watching for the relevant information, and you all did a great job with that!