Thanks for your report, Chambeco.
If you mean that you actually don't get to set aside and play Death Cart in your scenario, then this is the correct behavior according to the rules! Look at the following answer by Donald X., the game's creator, on BoardgameGeek
Setting the gained card aside ... it's just an instruction on Summon. You gain a card. Then you set it aside. All "when you gain this" abilities would happen prior to setting the card aside. And the card is not set it aside if it was lost somehow (e.g. covered up), which then causes the rest of Summon to fail due to the "if you do" (that the German version accidentally leaves out).
So you gain the Death Cart (to your discard pile). This triggers the Death Cart's own "When you gain this" ability, and the Ruins will cover the Death Cart up. So the game looses track of the Death Cart and can't set it aside.
This can be seen in the log as well:
Turn 3 - Ingix
I plays a Copper and 2 Silvers. (+$5)
I buys a Summon.
I gains a Death Cart.
I gains a Ruined Village.
Summon loses track of Death Cart (it was covered up).
I gains an Abandoned Mine.
I draws 3 Coppers and 2 Estates.
This is one of those cases where in real live there would be no confusion about 'where the Death Cart is', but the rules are implemented rigorously and you aren't supposed to play the Death Cart in this scenario IRL either, it's just that many players probably don't know this as it is unexpected.
It will work when there are no more Ruins to be gained with it, but otherwise I can't think of a way.