I found a bug: reaching water with Pebble with violetheart equipped from the surface for the first time does not unlock both relevant achievements; it only unlocks the hard hat achievement.
Disclaimer: tested in vegan mode; not in regular mode. Problem probably exists in both modes; and unlocking 3 or more achievements at once likely causes this same problem.
Thanks for the detailed bug report! I will look into this and try to track down the exact cause and a possible fix for it.
Edit:
I tested this on both hard mode and normal mode with the Violetheart Pendant equipped from the surface. In my runs, I unlocked three achievements in addition to Hard Hat.
"My Final Form" unlocked midway through the run when Pebble reached 100 stacks of Fortify. When I reached water, both "Dry Run" and "Fashion Statement" unlocked as expected. I have not been able to reproduce the issue so far.
