For anyone wondering how to fix the DefaultTimer, the ShooterGame simply added the following function:
virtual void PreInitializeComponents() override;
/** Handle for efficient management of DefaultTimer timer */
FTimerHandle TimerHandle_DefaultTimer;
void AShooterGameMode::PreInitializeComponents()
{
Super::PreInitializeComponents();
GetWorldTimerManager().SetTimer(TimerHandle_DefaultTimer, this, &AShooterGameMode::DefaultTimer, GetWorldSettings()->GetEffectiveTimeDilation(), true);
}