Tamiyo states, "While this support is on the board, whenever your opponent makes you discard a card, return that card to your hand. It gains half of its mana."
Nicol Bolas, Ravager’s second ability states, “Your opponent discards their first and last card. Increase the cost of cards in your opponent’s hand by 3.”
NB’s ability is overriding Tamiyo’s effect. When the AI activates NB’s second ability, Tamiyo’s ability seems to trigger and then the card is discarded, leaving you without the cards that were supposed to return to your hand.
It actually seems to happen with several (all?) discard effects, @madwren . I was hit with Awaken the Erstwhile, which is supposed to discard all your cards, and it did the same weird thing of triggering and then discarding anyway. As it stands, her continuous effect is useless. Perhaps edit the title to reflect this and get more attention to this?
I can confirm that the team is aware of and looking into this issue as well, but I am unable to confirm when the fix for this particular issue will be implemented at the moment.