Indexing packages that use this file extension (e.g. for their index.mjs) and adding them to the compiler environment is currently broken. As a result, these packages will not be resolvable at runtime, leading to errors such as this:
I've created a minimal repository to reproduce the issue here: https://github.com/Jannis/cljs-npm-deps-mjs-issue.
I'll attach a patch with a fix and two tests (one to verify indexing, one to verify building) shortly.
Tested with both ClojureScript 1.10.64 and ClojureScript master (4375a63)