My insight this evening is that it looks like there's a bug in triggering achievements.
I did multiple tests, starting with a fresh copy of Tidalis that lacked any achievements or play stats:
With Vinraith's puzzle, which should work perfectly fine, I was not able to trigger the achievement on multiple attempts. I decided to try failing the puzzle while still meeting the requirements for the achievement. By doing that, I was awarded the achievement on all three of my attempts. Meeting the requires for the achievement, winning, and then losing without meeting the requirements did not grant me the achievement, and the game stats show the depth of my deepest combo as 0, which can't be right at all.
When I won, I was given three achievements: the achievement for winning one round and the first two Score achievements, which are for a certain number of points from a specific chain. If I lose by not clearing one of last cyan blocks in his puzzle, I'm granted FIVE different achievements related to combos.
What I think is happening is that winning causes the stage to end before some logic has been done related to completing combos. Therefore, the game stats don't include that combo and the achievements are never given out. I don't recall what it was, but I remember that Keith hit some complexity with tracking combos in some fashion. Maybe he'd have some insight into fixing this?