Sorry, I missed the part about opening on a mine. I think I know where the problem is, and I'll try to fix it in the next patch.
Regarding the damage, there is also a boss modifier that increases it by 1, so it would be 2 damage for each mine. If you have the seed, I'll try to confirm if it was present or not. However, if exactly 7 mines exploded, it should have caused 14 damage, not 22. If it was 1 damage for each mine plus each adjacent cell, it would have been much more than 22. Graphically, it is correct that the adjacent cells are damaged; it is part of the malus due to the enlarged explosion effect.
I like the idea of giving weight to each boss modifier to make the resulting stage more balanced. I will work on it, thanks again for the feedback!