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Test neck flex

Sounds like the truss rod is doing what it's supposed to do: counteract the tension of the strings tuned to pitch.

So...when you slackened three of the four strings the remaining string went sharp, since the truss rod was pulling back against the tension of only that one string.

In terms of setup, dunno that observation gets you anything. The truss rod is adjusted to produce the neck relief you want, and that means it's adjusted with the strings at pitch.
 
I just wonder if anyone has done this and what the result is without adjusting anything. As I didn't test this on another bass, but on another bass I noticed I could flex the neck relatively easily fully stringed. Seems my bass neck is pretty strong to only tighten the string a little more than a half step with the others taken out.