What you hit is an effort to determine the alias of a FileResource from the URI provided during the lookup.
This is "best effort" approach, and is often at odds with the modern realities of the web and internationalization.
When we started the Jetty 9.3 branch (for http2) we switched from FileResource (based on java.io.File) to PathResource (based on java.nio.file) starting in Jetty 9.3
This also means a ton of these sorts of issues are no longer handled by us, but rather by built-in java classes.
We even deprecated FileResource in Jetty 9.3
The PathResource exists in Jetty 9.2 as well, but its not set as default behavior.
(Note: the version in Jetty 9.2 is an earlier implementation, but works well enough. The version in 9.3 is better, and is tested to a much higher level)
You can use the new PathResource in your WebAppContext by using the .setBaseResource(Resource) method on your context.
Please switch your Embedded Jetty to use context.setBaseResource(new PathResource(new File("/path/to/webapp")))
This might have the desired effect you are going after.
Another approach would be to copy the 9.3 implementation from our master branch into your project to use it.
This should help.