Fragment shader failing when accessing higher array indexed input component - opengl

Question

I'm trying to access an array of structs in a glsl fragment shader passed from a geometry shader, and I'm having odd failures with any attempt to access certain indices over an arbitrary number. I've already discovered you apparently can't access an array by variable in a fragment shader, but it fails even using an int literal.
I've reduced the shader down a simplified version that shows the problem. This receives a series of quads (triangle strips) from the geometry shader and renders red rectangles, with a smaller sub-rectangle in the middle being rendered yellow if the relative fragment falls into an arbitrary bounding box passed along from the vertex.
#version 400 core
struct SpriteLayer {
ivec2 texPos;
ivec2 offset;
ivec2 dim;
int texNum;
uint flags;
uint color;
};
in vec2 lmUV;
flat in int frag_numLayers;
flat in uint frag_totalColor;
in vec2 pixel;
flat in SpriteLayer spriteLayer[6];
out vec4 color;
uniform sampler2D texLightmap;
uniform sampler2D texBase[16];
uniform int numTextures;
uniform vec4 ambientLight;
uniform vec2 randomSeed;
void main() {
color = vec4(1f, 0f, 0f, 1f);
SpriteLayer lay = spriteLayer[1]; // works
//SpriteLayer lay = spriteLayer[5]; // fails if preceding line replaced with this
if (
(pixel.x >= lay.offset.x) &&
(pixel.x < lay.offset.x + lay.dim.x) &&
(pixel.y >= lay.offset.y) &&
(pixel.y < lay.offset.y + lay.dim.y)
) {
color.g = 1f;
}
}
I checked GL_MAX_FRAGMENT_INPUT_COMPONENTS and my environment reports 128. Unless I'm very mistaken in how that works, I only count around 60 total input components here? Just to check I also tried padding the struct out, but no change. Despite being an array of 6 structs, when I access elements spriteLayer[0] through [3], the shader renders, but when I change the code to access spriteLayer[5] it fails, with no compilation error and no rendering whatsoever, with my program unable to set the uniforms. Trying to access spriteLayer[4] sometimes succeeds and sometimes fails depending on how I restructure the code, yet this quirk doesn't seem dependent on the number of input components at all (!?). Same problem with direct access to the array without the intermediary SpriteLayer variable. What's going on here?