The ReleaseInfoPlugin part of jlink uses this code when reading from a file via "--release-info /path/to/file" option:

                // --release-info <file>
                Properties props = new Properties();
                try (FileInputStream fis = new FileInputStream(operation)) {
                    props.load(fis);
                } catch (IOException exp) {
                    throw new UncheckedIOException(exp);
                }
                props.forEach((k, v) -> release.put(k.toString(), v.toString()));

In other words, the input file when the release-info plugin runs is in UTF-8 (see JDK-8301971), while the Properties.load(InputStream) API assumes the input stream to be in ISO-8859-1.

This would be visible on a build with non-ASCII characters in the vendor string when building the JDK:

$ configure --with-vendor-name='ößäakl foo'
$ make images

It's best to use Properties.load(Reader) API instead to avoid this.