I was finally able to reset uniforms variable once on reshade_begin_effects event using this code :
Global variable definition :
static LONG g_uniformReseted = 0;
reshade_begin_effects callback :
if (InterlockedCompareExchange(&g_uniformReseted, 1, 0) != 0) return; // reset OSD uniforms once
if (<myuniformvariable>.handle && ...) {
runtime->reset_uniform_value(<myuniformvariable>;);
...
runtime->save_current_preset();
}