Don't use Progress Bar, it's a form control, they are difficult to style and have lots of problems.
Progress bar is ridiculously easy to make with a sprite or a tiled background object. Simply create a rectangular sprite, set image point position at x=0, and on every tick update its width.
For example:
HealthBar Set width max(maxWidth*(currentHP/maxHP), 0)
Note - max(...,0) expression is only needed to prevent it growing in the opposite direction in case the HP value drops below 0.