# Why are there these jagged "shards" in my RegionPlot3D?

I'm creating a region out of two component regions. Before anyone suggests it (because a lot of similar questions are about it), I don't think the problem is increasing PlotPoints. I've increased it to 100 in a few steps and the problem seems to stay.

hCub = 1.5;
W = 5;
L = 10;

Clear[x, y, z];
cylcenter = {W, L/2, 0};
region["cub"] =
ImplicitRegion[
0 <= x <= W && 0 <= y <= L && 0 <= z <= hCub, {x, y, z}];
region["halfcyl"] =
ImplicitRegion[
0 <= (x - cylcenter[[1]])^2 + (y - cylcenter[[2]])^2 <= AAOrad &&
0 <= (z - 0) <= hCub && x <= cylcenter[[1]], {x, y, z}];
region["thinshellhalfcyl"] =
shellthickness) <= (x - cylcenter[[1]])^2 + (y -
cylcenter[[2]])^2 <= AAOrad && 0 <= (z - 0) <= hCub &&
x <= cylcenter[[1]], {x, y, z}];
region["cubNoHalfCyl"] =
ImplicitRegion[{x, y, z} \[Element]
region["cub"] && {x, y, z} \[NotElement] region["halfcyl"], {x, y,
z}]
RegionPlot3D[
{region["cubNoHalfCyl"], region["thinshellhalfcyl"]},
PlotPoints -> 100,
PlotStyle -> {{Black, Lighting -> "Neutral",
[email protected]}, {Lighting -> "Neutral",
Gray}},(*BoundaryStyle\[Rule]{None,Black},*)Boxed -> False,
ImageSize -> 800]


Here's what that produces, from two different angles:

As you can see, it's filled with these ugly weird shards. If I plot just the box, you still see them:

RegionPlot3D[Evaluate@region["cub"],
PlotStyle -> Directive[Black, Lighting -> "Neutral", [email protected]],
PlotPoints -> 100]


I tried setting PlotPoints->500, which seems like it shouldn't be necessary. It's still trying to evaluate that for just the box example, so if it completes and looks better I'll amend this.

What's going on?

I associate the shards with wrongly oriented polygons or some other such glitch. Instead of tracking them down, I tried doing a plot with a function instead of a region, and it seemed to work.

rp1 = RegionPlot3D[
RegionMember[region["cubNoHalfCyl"], {x, y, z}],  (* region function *)
Evaluate[                                         (* create the domain specs *)
Sequence @@