It is subtly translucent.
It would be easier to understand if you look at the feathers displayed by EssentialMod.

This problem does not seem to occur with the latest IrisShader. Maybe.

Yeah I'm aware of this bug, I'm just not sure what's causing it yet

Since the HealthBar is translucent when it is displayed, I think that maybe there is something mixed up in the HealthBar code that makes the player translucent.

It does this with item frames and even mobs too.

After trying various mods, it seems that this problem occurs when I start with IrisShader in.

Yeah it's definitely a shader problem
It's why I haven't figured out the actual problem, because shaders break all sorts of stuff, and TES's code looks fine for this

Should I submit this to IrisShader?

Typically they're not very receptive to bug reports that involve any other mod
Or helping with issues and stuff

But you can try I suppose?

I decided to give it a shot.

In the 1.21.5 version, this problem was resolved, but with shaders enabled, the bars were no longer displayed, or the bars were not displayed in the shadow areas of the numbers.