This seems to have just started with the latest update. I tried to add the final fragment to a constellation and couldn't select it. I closed out the game and reopened it and still had the same problem. I was finally able to select the constellation by clicking off to the side of where it was on the screen.
I tested clicking in different areas around the constellation map and repeatedly had it select constellations that were not the one under the mouse cursor. It even selected a constellation that was completely off screen at one point.