The vertex shader input variable gl_InstanceIndex should count from the base instance, i.e.

    base, base+1, ..., base+count-1

see https://github.com/KhronosGroup/GLSL/blob/main/extensions/khr/GL_KHR_vulkan_glsl.txt , line 580.
This works as expected with the Vulkan backend. But with the D3D11 and D3D12 backends, gl_InstanceIndex incorrectly counts from 0, i.e.

    0, 1, ..., count-1 .

I have written a bare bones example program that demonstrates the issue.
It draws a single quad using the command

    commandBuffer->draw(4, 1, 0, 100);

The vertex shader then sets the color as follows

    if (gl_InstanceIndex == 0)
        fragColor = vec3(1.0, 0.0, 0.0);
    else if (gl_InstanceIndex == 100)
        fragColor = vec3(0.0, 1.0, 0.0); // expected
    else
        fragColor = vec3(0.0, 0.0, 0.0);

The expected behavior is to get a green quad.
But with D3D11 and D3D12 I get a red quad.

The code can be downloaded from https://github.com/jrade/InstanceIndexIssue .
The RHI backend can be set in the main function.
The shaders can be baked with the script Resources/BakeShaders.bat.

The code was tested with Windows 11, Visual Studio 2022, Qt 6.9.2
and both an NVIDIA GeForce and an Intel Iris GPU.