https://forums.unrealengine.com/t/how-to-call-c-function-from-blueprint/453335
Function Libraryの関数は静的の必要があるらしい。
先頭にstaticとつけリビルド。
UFUNCTION(BlueprintCallable, Category = "LoadingScreen")
static void SetupLoadingScreen(UUserWidget* LoadingScreen, float MinimumDisplayTime = 3.0f);