When you mixin into an inner class, the refmap will have the intermediary name, in both Forge and Fabric (and not srg in Forge)

[natanfudge]

Like here

@Mixin(targets = "net.minecraft.util.crash.CrashReportSection$Element")
public abstract class CrashReportSectionElementMixin {

The refmap will look like this (IN FORGE)

    "fudge/notenoughcrashes/mixins/CrashReportSectionElementMixin": {
      "net.minecraft.util.crash.CrashReportSection$Element": "net/minecraft/class_129$class_130"
    },

While everything else is properly mapped to srg:

    "fudge/notenoughcrashes/mixins/client/SplashScreenMixin": {
      "LOGO": "field_212973_a:Lnet/minecraft/util/ResourceLocation;"
    }