Hi all.
I use Is Dedicated Server blueprint but it doesn’t work with real dedicated server.
It returns false.
It seems it only happens in game instance Init event.
I can confirm the same issue at the moment, Trying to sort a work around using a game.ini variable. Again only on the GameInstance Init. Obviously it does’nt know its a dedicated server yet, Might find a way to make it ask the question a little later after init. Would be nice to know exactly where in the Initialization chain its sets the “Is Dedicated Server” bool.
The dedicated server flag is set just after the init call to the blueprint. 
My current solution is to call the relevant functions in GameInstance from the GameMode begin play event, as that will happen after.