The compiler seems could auto load the MixinConfigs in the version below 1.20.6, but it didnt work from 1.20.6. I just modified the jar file to zip and found the META-INF/MANIFEST.MF is lack of "MixinConfigs" compared with the lower version, while the build.gradle are same. So I manual added it to fix the problem.
I dont know the implmentation of this process, but a different is in 1.20.6 the jar task deleted the reobfJar task, maybe this leads the problem?