I experienced the following UI interaction, which I believe is not an expected behaviour when handling Rottenmouth Viper's ability.

1. My opponent attacked me with Rottenmouth Viper with one blight counter on it.
2. Another blight counter was added, making 2 counters total.
3. Then, MTG Arena asked me my option (with totally broken Japanese message, though I know what happens and didn't waste time to try to read that message).
4. I decided to discard two cards from my hand (I had four then) in my mind.
5. I chose two cards in my hand for discarding, but the orange button on the lower right was disabled.
6. I found that the button can become clickable if I choose only one card in my hand for discarding. I chose one card in my hand and clicked the button.
7. Then, Arena sacrificed my only untapped creature at the same time as discarding the card I chose without asking me an option to discard another card. (I had another creature that was tapped on the battlefield, by the way.)

I don't know whether the observed behavior is reproducible or not.