I just came here to report the same bug.
I'm on an iPhone 5S and this happened multiple times in the same game:
1. Defeat Foxglove lady.
2. Find Skinsaw Man in The Warrens and put him on top of the Warrens location deck.
3. My rogue proceeds to encounter him that turn. She's alone in the location, so he gets his 1d4 increased difficulty plus the -5 difficulty for defeating the Foxglove lady.
4. First phase goes as expected.
5. Second phase he has a difficulty of 27+. I got close to beating him here once thanks to backstab etc but never managed it, so I was always out of cards for...
6. There is a third phase with the difficulty one would expect for the second phase.