Nope, it's something that happens from time to time. I believe the frequency goes up with the difficulty level increases, but I can't recall if that is true for sure. At any rate, it's been a fun feature of the game since 1.0 and before, heh.
Having a Mark IV world gives you a couple of challenges/opportunities:
1. Starting out, that thing will be weak in numbers, even though it is Mark IV. You can take that over if you do it right. Since there are a fixed number of Mark IV planets in the galaxy depending on your map size and difficulty level, that actually comes out for you as a net win versus running into a heavily-reinforced Mark IV planet later in the game. So there is an opportunity here.
2. Alternatively, you can just use transports to skirt through there, and make sure you destroy the warp gate along the way. Then this is your own personal little bugbear waiting in your backyard, but you can probably take it out later. This is more the challenge side. Even so, it's one fewer Mark IV worlds somewhere else that you might care more about, so that can be positive.
Welcome to the forums, by the way!