I'm pretty sure you were right. That it should have made him -1 life even though the card was tapped. I was positive at first but I know I've seen that happen before with +1 life a few times. Here's what I experimented with after reading your predicament.
Now I don't play the actual card game but I have been playing this on Steam for the past year. For the most part, rules are the same for both versions of the game.
I used this
Both of the Decks in this game were green. Made sure I had him tapped and let the other player play a card and resulted in this
the opponent played the 2/3 card at the top of the screen and my card's counter went from 8/8 to 9/9 while it was tapped. Regardless of it being a +1 counter, I'm sure + or -1 health works the same way when it's tapped.
Hope that helped..
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