Hearthpwn

I had multiple Arena runs where I had Runes of Darkness in my deck last week and in all those runs after I played the card, it did not discover anything.

I had opponents playing that same card just fine.

Does anyone else have had this bug? It's very annoying, cause it might have costed me several losses.

Did you play a class with no weapons in the current arena rotation?

Ah so that's it. I was playing mage/death knight. 

But if that's the case, then they should not let you give the option to pick that card when playing mage. Ban it, when you don't play the main class.

Yeah.  Or… you could just understand the interaction and not pick a dead card.

Except it is a bug, as class cards that would offer you no discoverable cards due to the class that you are in will instead offer you three options from the class of the card itself. This type of situation can happen in regular Hearthstone due to randomly generated cards or in dual class Arena. As an example, if you are playing Priest main and Hunter secondary in Arena right now, and cast Titanforged Traps, you will get offered three Hunter secrets, as Priest doesn't have any secrets. However, if you cast it while Mage is your main class, you will get offered Mage secrets.

The other oddity that can happen is when a class has three or fewer options, and then you will get the same options every time. This happens frequently now with cards that discover demons (Blind Box, Demonic Dynamics, Window Shopper) when played by a class that doesn't have any class demons. In that case you always get offered the same three options-Adaptive Amalgam, Observer of Mysteries, and The One-Amalgam Band.

TLDR, report the bug.

Quote from Thonson >>

Yeah.  Or… you could just understand the interaction and not pick a dead card.

Like elevenx67 explained above my post, it is a bug. So instead of blaming people to understand the interaction, maybe just think about other interactions yourself and realise the game has bugs to fix.