As demonstrated here, the health bar appears below any overlays (most notably the nether portal overlay, but also vignettes).

This has been an issue in the mod for a while (I suspect since 928c8ae, though I haven't tested), but I haven't quite figured out why or how to fix it.

I don't know how I only see this now, but I'll look into this soon, thank you !

Hello
Today I had time to work on it, so I fixed this, I made a couple other changes so it fits well.
If you want to check the new version on modrinth : https://modrinth.com/mod/simple-health-bar/version/J2OYBazD
Otherwise the code is also here on github.
Thank you for your help !

This has been fixed in my fork at 82a58ed -- it was actually quite a simple fix, by just Redirecting the original health bar's invoke to the new healthBar.render() function. Blends behind chat and works with overlays!