isSigned returns true if normal JSON provided or singed with different key

I've two problems with the boolean isSigned(String jwt) method:

The following function call returns true if I provide a normal JSON (NOT a signed JWT):

Jwts //
	.parserBuilder() //
	.setSigningKey(getPublicKey()) //
	.build() //
	.isSigned(json);

If I change the method calls to the following:

Jwts //
	.parserBuilder() //
	.setSigningKey(getPublicKey()) //
	.build() //
	.parseClaimsJws(json);

a io.jsonwebtoken.MalformedJwtException: JWT strings must contain exactly 2 period characters. Found: 14 is thrown (which is the expected behavior).

As json a valid JSON is provided (NOT a JWT, maybe it can be any string?), e.g.

{
  "version": "foo",
  "url": "bar",
  "release_notes": "baz"
}

If I provide a valid JWT, signed with a different private key, isSigned also returns true.

From the doc:

* Returns {@code true} if the specified JWT compact string represents a signed JWT (aka a 'JWS'), {@code false}
* otherwise.
* <p>
* <p>Note that if you are reasonably sure that the token is signed, it is more efficient to attempt to
* parse the token (and catching exceptions if necessary) instead of calling this method first before parsing.</p>

Do I understand this method in a wrong way? I just want to check if a string is a JWT signed with the corresponding key. In my opinion isSigned should return false in both cases.

At a different code location I use parseClaimsJws, that works great

As version I use the latest 0.11.1