Note: Before responding with comments regarding the invisible card glitch being a known bug, understand that that bug is player-based while this is AI-based and seems to have a completely different cause.
At first I thought I was imagining things when I saw the AI discard cards from its hand without it being full, but now I have proof that I'm not completely crazy.
Observe the first picture:
Things to note:
1) The Arena deck has over 60 cards, meaning it should draw 2 cards next turn.
2) This is during my turn.
3) The AI has played at least 2 Shard of Serendipity by this point.
Now the next picture:
Things to note:
1) This is IMMEDIATELY after the AI drew from its deck, meaning 0 cards were played.
2) The AI drew 1 card when it should have drawn 2.
As you can see, the AI appears to have 2 invisible cards in its hand which is limiting the amount it can draw. It also discarded a card the turn before, but for obvious reasons I couldn't take a screenshot of that.
Even now I am still not sure what the cause of this bug is, however I am almost certain that it has something to do with Shard of Serendipity being used by the AI. More testing is needed before confirming this.
This also appears to be similar to the bug reported here (
http://elementscommunity.org/forum/index.php/topic,34900.0.html) in which case there may be multiple causes of this bug.
EDIT: After the turn above had passed, the AI played a 3rd Shard of Serendipity which DID allow the AI to add 3 more cards to its hand, but it still was not able to draw more cards from its deck until its hand size was below 6 cards. Strange.