GLSL (user defined) function parameters and uniform blocks strange behavior - opengl

Question

I am writing simple deferred rendering 'engine' for fun and encountered a strange behaviour with GLSL. Calling a function caused a slight malfunction while simply pasting the content of the function solved the problem (details below).
Am I doing something terribly wrong or is it possible that I hit a limitation or a glsl compiler bug (14.12 AMD Catalyst Omega drivers) ?
Original call (in a for loop over shadow):
ColorOut.rgb += phong(position.xyz, normal, color, Shadows[shadow].position.xyz, Shadows[shadow].color.rgb);
Using this, the color of my shadow casting lights are all the same, Shadows[shadow].color.rgb seem to always be equal the to first one of the array (in the phong function at least).
My 'solution' was to replace the call by the content of the function:
vec3 L = normalize(Shadows[shadow].position.xyz - position.xyz);
float dNL = dot(normal, L);
float diffuseFactor = max(dNL, minDiffuse);
vec3 V = normalize(cameraPosition - position.xyz);
vec3 R = normalize(reflect(-L, normal));
float specularFactor = pow(max(dot(R, V), 0.f), 8.0);
ColorOut.rgb += diffuseFactor * color * Shadows[shadow].color.rgb + specularFactor * Shadows[shadow].color.rgb;
With this, everything works fine.
Other related code snippets, shadow casting lights data structure:
layout(std140, binding = 2) uniform ShadowBlock
{
vec4 position;
vec4 color;
mat4 depthMVP;
} Shadows[8];
The phong function:
vec3 phong(vec3 p, vec3 N, vec3 diffuse, vec3 lp, vec3 lc)
{
vec3 L = normalize(lp - p);
float dNL = dot(N, L);
float diffuseFactor = max(dNL, minDiffuse);
vec3 V = normalize(cameraPosition - p);
vec3 R = normalize(reflect(-L, N));
float specularFactor = pow(max(dot(R, V), 0.f), 8.0);
return diffuseFactor * diffuse * lc + specularFactor * lc;
}