Yes, that's where knockout/overprint is set. Except in your example you're on Cyan, which allows you to change it from one to another. Click on White and your ability to change it is gone. It defaults to knockout. Except this specific text wasn't knocking out. When I preflight the pdf in Acrobat, it is clearly set to overprint.
Fortunately, I resolved this one myself this morning. And for anyone who may find themselves staring at invisible white text on a RIP'd file that the color palette will not let you change, the solution is fairly straight forward. Most of the text that I am referring to is legacy text. Been around, formatted and being used for many generations of Quark. Back as far as Q8 or earlier. Pickup catalog layouts and the like being the savior for much of the work in a high volume, deadline oriented agency. Trouble is, as of Quark 10 the trapping functionality went away. And having only now updated everyone's Quark to 2016 (did I mention we're a high volume outfit?) this little ditty just started happening.
By changing the text in question to another color, that overprint/knockout part of the colors palette turns back on. In every instance that I have found where this disappearing text occurred, the (now) colored text switches to overprint as soon as I apply the different color. Changing it to knockout as the other color, then returning it to white text, restores the text in the RIP'd files. So, yes, thank you max.peter, your suggestion is part of the solution. There's just a bit more to it.