Quick answer: Unreal Material static bool pin's default false overriding the material instance's true? Static pin defaults reset on master compile; instance values lost.
Master has bUseFancyEffect with default false. Material instance sets true. After master recompile, instance shows false.
Set default true if expected
If instances usually want true, set master default true. Instances overriding false; rare.
Or use dynamic param
Convert to dynamic scalar (0/1). Persists per instance; doesn't reset.
Re-save instances post-master
Editor utility: walk instances; re-apply overrides. Tedious; works.
“Static defaults can override instances on recompile. The override is silent.”
Avoid static bool pins for instance-tunable values. Use dynamic scalars; defaults survive.