As, Unfallen I built an explorer-type ship with only probes and engines (I just send it around to explore curiosities), and had the diplomatic immunity card unlocked. It got attacked and I used the card, expecting it to be undamaged, but this did not happen.

Isn't it supposed to? I had 0 attack (iirc, the card says 0 attack ships get 100% damage absorption) so it should have.

Of course, as I write this I realize that this would break the game (by having an unbreakable siege line), so maybe this should got into the balance suggestion forum, but technically the card is not operating as it says so I guess it's more of a bug.

Hi,

It's a wording issue: +100% damage absorption means that if your defenses would have absorbed 30% of a salvo damage, they will absorb 60% with this card.

We will try to clarify that, thanks!

AHHHHH. Okay, that makes much more sense balance-wise and text-wise. 

I would also suggest changing the name of the card? Maybe Diplomatic Escort or Security? 

Given that it gives an additional 100% bonus to the current damage absorption and it has immunity in the title, it gave me the immediate impression that the result of the text would being damage immune. Removing the word immunity from the title (or even changing the absorption  value so that it is not 100% even if it were something like 99.999%) would be the clearest way to make sure people understand the card correctly.