Oh sorry I did not understand your cuestion however its not a bug if you put it Transparent its going to have 0 opacity if you want to edit that you have to edit the value of opacity your code is the same as
Plot[-(x + 3)^2 + 2, {x, -10, 10}, PlotRange -> {-10.1, 10.1},
PlotStyle -> Opacity[0], AspectRatio -> 1, ImageSize -> 350]
To change that you have to put something like
Plot[-(x + 3)^2 + 2, {x, -10, 10}, PlotRange -> {-10.1, 10.1},
PlotStyle -> Opacity[0.3], AspectRatio -> 1, ImageSize -> 350]
or something like that