if we're being honest, vocal regressions are much more common in kpop than improvements. A lot of idols where much better at debut because they had the trainee period where they got to efficiently work on their vocals. But after some time in their career idols regress due to overworking their voice, straining, fatigue etc.
So which idols' technique regressed with the years?
On top of my head:
Hyorin (altho she bounced back from her regression, she fell significantly in late sistar years)