Ok nice. But curios to know with this configuration is it really compressing the JS as part of minification?

Suggested to create an Adobe AEM support ticket or you can try minify as part of your codebase build release as a workaround solution. We had tried these language options for ES6 javascripts but had come across similar issues.

Hi,If there are any compilation error seen in the AEM error log, try fixing it. If there are no error then just try with option min:gcc and no other language parameter. To recompile in the AEM instance put a space and save the original JS source file GCC option will try to compile and minify it. 

I am too facing the same issue. Here it is from log. 10.06.2020 14:40:34.526 *INFO* [202.174.120.2 [1591800034525] GET /etc.clientlibs/appName/clientlibs/clientlib-base.min.css HTTP/1.1] com.adobe.granite.ui.clientlibs.impl.HtmlLibraryManagerImpl Start building CSS library: /apps/appName/clientlibs/...