If you use the oltu JWSReader with a character encoding that is not single-byte, like UTF-8, the signature is parsed incorrectly.
This is because the signature, according to the JWT spec, is a base64url encoded byte array (any byte, generated by HMAC256), which is now decoded directly into a String. If the character encoding is set to UTF-8 (-Dfile.encoding=UTF-8), the decoded signature will be a different byte array, usually longer. If the character encoding used is something single-byte, like MacRoman, this happens to work.
From the JWS spec:
Create the desired serialized output. The JWS Compact Serialization of this result is BASE64URL(UTF8(JWS Protected Header)) || '.' || BASE64URL(JWS Payload) || '.' || BASE64URL(JWS Signature).
The solution is to change the JWS reader to store the signature as base64url and compare that.