Current StringJoiner code tries to build the char[] storage by itself:

    @Override
    public String toString() {
        ...
        final String delimiter = this.delimiter;
        final char[] chars = new char[len + addLen];
        int k = getChars(prefix, chars, 0);
        if (size > 0) {
            k += getChars(elts[0], chars, k);
            for (int i = 1; i < size; i++) {
                k += getChars(delimiter, chars, k);
                k += getChars(elts[i], chars, k);
            }
        }
        k += getChars(suffix, chars, k);
        return jla.newStringUnsafe(chars);
    }

This is seems to be a performance optimization, but it clashes with Compact Strings which now have to re-compress the resulting char[] array into byte[]. We may want to extend this mechanism by figuring out the coders for arguments, and creating byte[] with appropriate coder, then using a private String constructor that does not re-compress.