Option Lighting in the Show function does not work

Question

Bug introduced in 11.2 or earlier and persisting through 12.1

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There are something wrong when I try to evaluate the example from Lighting Docs, here is the code:

lights = {{"Spot", Red, {{3, 3, 5}, {3, 3, 0}}, Pi/8}, {"Spot", Green, {{7, 3, 5}, {7, 3, 0}}, Pi/8}, {"Spot", Blue, {{5, 6, 5}, {5, 6, 0}}, Pi/8}}; 

plane = ParametricPlot3D[{u, v, -2}, {u, 0, 10}, {v, 0, 9}, PlotPoints -> 100, MaxRecursion -> 0, Mesh -> None, Axes -> False];  

Show[plane, Lighting -> lights]  

Unfortunately it does not work as expected in Mathematica version 11.3.
But if we directly use the form

ParametricPlot3D[{u, v, -2}, {u, 0, 10}, {v, 0, 9}, PlotPoints -> 100, MaxRecursion -> 0, Mesh -> None, Axes -> False,Lighting->lights]  

it works.

So I want to know the reason for creating the different outcome, may be researching the InputForm of output seems a feasible point, but I failed. Thanks for any info on this matter.

Answer 1

Update: This is yet another case of automatic PlotTheme settings injecting directives that override the user-specified ones. So, a much easier fix is to add the option PlotTheme -> None to ParametricPlot3D.

plane = ParametricPlot3D[{u, v, -2}, {u, 0, 10}, {v, 0, 9}, 
   PlotPoints -> 100, MaxRecursion -> 0, Mesh -> None, Axes -> False, 
   PlotTheme -> None];

Show[plane, Lighting -> lights]

Note: You can find a number of other issues fixed by explicitly removing PlotTheme on this page.

Original answer:

I think this is a bug.

A fix is to remove Lighting from plane before using it in Show:

plane = ParametricPlot3D[{u, v, -2}, {u, 0, 10}, {v, 0, 9}, 
   PlotPoints -> 100, MaxRecursion -> 0, Mesh -> None, Axes -> False, ImageSize -> 300];
planeb = DeleteCases[plane, Lighting -> _, All];
Grid[Labeled[ToExpression@#, #, Top] & /@ # & /@ 
 {{"plane", "Show[plane,Lighting->lights]"},
  {"planeb", "Show[planeb,Lighting->lights]"}}]

What I think is happening:

The lighting settings from the default PlotTheme -> Automatic are embedded as directives before graphics primitives in ParametricPlot3D. This can be verified using

Cases[plane, {a : ___, l : (Lighting -> _), ___} :> {a, l}, All]

or using

Cases["DefaultPlotStyle" /. (Method /. 
  Charting`ResolvePlotTheme[Automatic, ParametricPlot3D]), HoldPattern[Lighting -> _], All]

As noted by xzczd in comments, this issue does not arise in version 9:

Labeled[Row[Labeled[ToExpression@#, #, Top] & /@ {"plane", 
    "Show[plane,Lighting -> lights]"}], "Version: " <> $Version]