I also bought one of these recorders recently, so just now I unboxed it and replicated your setup. To my astonishment, the record level controls are in effect even when the input is digital. And I don't see a way to defeat or bypass them. (Approximate unity gain is near the top of their range, but it's only approximate.)
If I'd known this I wouldn't have bought the recorder, though I may use it for its analog inputs at some point. Meanwhile, unless/until there's a workaround, I wouldn't use it as a "bit bucket".
I mean, I can see the designers' point of view. But even if they wanted to make this behavior the default for the benefit of less technically informed users, the controls really should be "bypassable" via a menu item.
--best regards