What’s happening is that the alpha parameter on PART 2 is defaulting to thisRow.alpha instead of currentValue.alpha.
So that should be straightforward. But here’s the funny part: if you type currentValue.alpha, you’ll see it returns the full alpha column. This is because somehow currentValue is being evaluated to the full colors2 table. wtf?
I had tried both thisRow.alpha & currentValue.alpha – this was the first time that I’ve noticed both contexts offered in the code-complete.
I didn’t notice that currentValue was evaluating to the table, but upon closer read of the prompt in the formula editor that does appear to be the case.
Thanks for the report. Unfortunately the column parameter of sort only works today if the input at the start of a chain is a table. This makes more sense when your input is a list of numbers or words, but gets particularly confusing when the input is a list of atReferences. We should totally make this work, but sadly isn’t the case today.