I think the setting is called "pure hue" in Onyx, and maybe something similar in Flexi. It forces the RIP to keep anything that's set to 100% CMYK values, without mixing in additional colors. In most cases it's best to leave that off, and let the RIP do it's thing.
Unless you know about it, it can be frustrating when the it looks like the RIP isn't keeping your small black text a single channel. But like we mentioned, any RIP generated elements are often 100%, so that's why it can be funky to figure out what's going on when they're printed right next to each other.