I'm very happy to fix the API bug, but I can't start it.

The "API bug" wasn't a bug, there just wasn't any reason for having @JvmField or @JvmStatic there anymore, after all, here is CCBlueX team's thinking process: scripts were not intended to use LB utils, therefore LB utils don't have to support scripts when being manipulated with. I wanted to make stuff compatible, but I agree that putting
@JvmField and @JvmStatic everywhere is not a solution. Therefore, scripts will have to adapt to code changes and no compatibility will be made. Actually, I just voluntarily broke all Core scripts probably, not mentioning the base itself.

It is possible, that possible built-in script objects that will return calls from LB utils might get added. You therefore wouldn't have to import stuff every time you just want to change targetRotation...

Now regarding the "vanilla" situation.
The cause of this problem is:
[13:50:29] [main/ERROR] [mixin]: Error encountered reading mixin config liquidbounce.forge.mixins.json: java.lang.IllegalArgumentException The specified resource 'liquidbounce.forge.mixins.json' was invalid or could not be read
There were major backend changes which might have caused this, however, this makes no sense to me, when I build the identical branch and run it, it works without any errors, same with running from IntelliJ.