Been away for a while, and I started a new game as UE to try the new build. On my very first turn, I was mousing around for info on the home system when I saw this notice on the migration icon:

"Populations from Pavo are immigrating to this system. Ships containing one population at a time will arrive regularly."

Well that's nice, but I've never met these Pavo system guys. It's my first turn and I haven't moved out of my home planet yet! 

Is this a bug or working as intended? Maybe tourists that have heard of us somehow. They might not like what they find when they get here, it's kind of a dump.

Probably a bug.  

I don't see an issue with population coming in from unknown systems in terms of it being believable later in the game.  You don't always get to explore everything or beyond another empires with closed borders.  That doesn't mean that the worlds in the galaxy don't know of your great empire as one of the major players in the universe and possibly attract migration.  

I guess that can apply even on turn one for bordering systems but it just really shouldn't.  An extra population that early is a bit too unbalanced so I'm guessing it's a bug.  An easy fix would be to just make migration coming from only explored systems.  I think it would be a fun mechanic though, if you have happy/developed/leveled up systems, that many people of the galaxy would flock to your empire.  

I liked Stellaris' take on it.  If you have open borders with an alien race and you have an empire that was attractive, you could have populations from other empires asking to immigrate. It was a cool mechanic.

It did remind me of the Stellaris mechanic, just way too early in the game, before I've got anything a visitor would be interested in. 

Another possibility would be very early contact from refugees fleeing something really bad in their home system, and you're the only safe haven in the area. Which is a cool idea, but probably very unbalancing if you get free pops that early.