From the code in:
https://github.com/TeamWizardry/LibrarianLib/blob/1.12/src/main/java/com/teamwizardry/librarianlib/core/client/LibClientProxy.kt#L81
It seems to be intended that TextureMapExporter is run only in an obfuscated environment, not all the time. It runs regardless of this variable, because it is subscribed with an annotation:
https://github.com/TeamWizardry/LibrarianLib/blob/1.12/src/main/java/com/teamwizardry/librarianlib/core/client/TextureMapExporter.kt#L27

The fix for this would be to manually register the class in LibClientProxy rather than with an annotation.