The screenshot is self-explanatory

:3

Also added logs: 2020-07-05-1.log

It's caused by OptiFine, should be pretty easy to fix so I guess I'll do that (at some point)

Meanwhile you can use this instead of my mod as a dirty workaround

A 1.16 snapshot (20w22a) had the same issue because the way the bar was rendered before was with 3 rectangles on top of each other (a black one for the outline, then a white one (same color as the vanilla background), and then the red one), and it was changed to 4 1-wide black rectangles and a red one, and I think OptiFine just reverts that change

Anyway, using the old mixins when optifine is present should fix it, haven't tested yet but it should be working

I found that it thinks that the left thingy is the bar color

https://github.com/A5b84/dark-loading-screen/blob/master/src/main/java/io/github/a5b84/darkloadingscreen/mixin/SplashScreenMixin.java
So maybe putting the Bar color index and the Optifine color index and give appropriate value depending on if Optifine is loaded or not

Also alternatively you could do a hax to inject assets/minecraft/optifine/color.properties dynamically if Optifine is loaded

For now, I put the bar color the sames as the border color :3

Turns out that OptiFIne actually calls fill one more time that vanilla and it doesn't appear to be visible (it's the white one, maybe it's for the bar background but I'm too lazy to check ¯\_(ツ)_/¯ )

Anyway, easy to fix so expect a new version soon :>