return TMap get undefine result

Development Programming & Scripting
,
1

Here I have a TMap:

// .h
private:
	static TMap<FGameplayTag, FGameplayTag> DamageToResilienceTypeTags;

//.cpp
TMap<FGameplayTag, FGameplayTag> UAuraBlueprintFunctionLibrary::DamageToResilienceTypeTags = TMap<FGameplayTag, FGameplayTag> 
{
	{AuraGameplayTags::Damage_Fire, AuraGameplayTags::Attribute_Secondary_ResilienceFire},
	{AuraGameplayTags::Damage_Lightning, AuraGameplayTags::Attribute_Secondary_ResilienceLightning},
	{AuraGameplayTags::Damage_Arcane, AuraGameplayTags::Attribute_Secondary_ResilienceArcane},
	{AuraGameplayTags::Damage_Physical, AuraGameplayTags::Attribute_Secondary_ResiliencePhysical},
};

Then, I have a Getter function for it:

//.h
UFUNCTION(BlueprintCallable)
static TMap<FGameplayTag, FGameplayTag> GetAuraDamageToResilienceTypeTags()

//.cpp
TMap<FGameplayTag, FGameplayTag> UAuraBlueprintFunctionLibrary::GetAuraDamageToResilienceTypeTags()
{
	return DamageToResilienceTypeTags;
}

At runtime, this Getter function get undefine results like this:

image
image1246×188 35.2 KB

Question:
1. Why is that, any problem about my code?
2. When I modify Getter function(use reference return), problem solved, why is that?

//.h
UFUNCTION(BlueprintCallable)
static TMap<FGameplayTag, FGameplayTag>& GetAuraDamageToResilienceTypeTags();

//.cpp
TMap<FGameplayTag, FGameplayTag>& UAuraBlueprintFunctionLibrary::GetAuraDamageToResilienceTypeTags()
{
	return DamageToResilienceTypeTags;
}