I've started playing only a couple of days ago so not sure if this is a bug or not.
I spied out a colony, it had tons of resources and no garrison. I go to assault it, but as soon as my fleet arrives at the colony it just returns to base. Just to make it clear: this is not on a starter planet.
Hi feskslo, if you assult someone's colony, and you win. your fleets will garrison in his colony. You can take some of his resources or establish trade routes. But if your fleets return to base, i am afraid you fleets may be defeated without your notice. did you check?
There was no garrison at the colony in question (checked before and after the assault!). There was no fight, I received no message with battle report. My fleet just turns and heads home when it arrives at the colony, no ships lost. They did not garrison the colony. I checked to make sure that I had room for more trade routes at my colony. I also sent a different fleet to check if my first fleet was bugged. Same problem. Also, both fleets could garrison different colonies afterwards. So it seems the colony was bugged. I can PM you the name of the bugged colony if that helps.
With a shielded colony, the "assault" and "obliterate" buttons are greyed out, and you can't send the assault to begin with. Also, there's a countdown timer above the colony showing for how long the colony is shielded.
But if you send assault, and THEN the player assaulted use a shield, it happens what you described. As I don't know exactly what happened, and that this "shielding" thing happened to me, I just though maybe it was the same for you. Or maybe a bug related to this.