3D Surfaces Not Rendering

I have two expressions that I want to use to display a surface in the 3D Graphics pane:

The values of x, y, and z are restricted to the domain [0, 1].

It is trivial for GeoGebra to take any (x, y, z) point and output a value for L. But here is where I run into trouble: I would like to plot a surface that represents all (x, y, z) points with a given L value.

If I input something like 0.2126 T(x) + 0.7152 T(y) + 0.0722 T(z) = 0.6, nothing appears in the Graphics or 3D Graphics panels. I don't get any error message, and the Algebra panel does show that an Implicit Surface has been created from the expression and does not show it as being undefined, but nothing is visible

Going to the Advanced Preferences for the Implicit Surface only shows checkboxes for viewing the Implicit Surface in the Graphics or Graphics 2 panels, with no option for the 3D Graphics panel. Checking the boxes for either of the 2D graphics panels does not cause anything to appear in them, not even the intersection of the supposed surface with the XY plane.

Inputting something like 0.2126 T(x) + 0.7152 T(y) = 0.6 produces an Implicit Curve that does appear in all three Graphics panels, with three checkboxes in its Advanced Preferences. I can even make a dynamic Implicit Curve by setting it equal to a Number object whose value is the output of L with sliders as inputs, and everything works fine.

But surfaces, static or dynamic, seem to be a no-go.

I've attached a GGB file with the basic setup for reference. Is there something I'm missing here, or is this a bug?