GLS Pure virtual function being called always appear

Hello, why Pure virtual function being called appear so often?
I only use CPP code and in one case the callstack brings me to an Insert in a TSet<>.
Its a simple “Owner->SelectedObjects.Add(Curr):” Where SelectedObjects is just a TSet of UInterfaces, it does not appear everytime, i would say 10% of the times but I cant really tell what cause it, it’s also hard to debug since it’s not super frequent…