Special characters in matrix parameter like slash "/" are not encoded.
Thus the slash interpreted as the beginning of a new segment.

UriBuilderImpl uriBuilder = new UriBuilderImpl();
uriBuilder.matrixParam("matrixParam", "12/3");
uriBuilder.build();

The resulting matrix parameter is "12/3" and not "12%2F" as expected.

Possible Bugfix: initialize matrixParameterEncoding from pathSegmentEncoding so that "/" is added to the list of characters to encode.
static
{
...
System.arraycopy(pathSegmentEncoding, 0, matrixParameterEncoding, 0, pathEncoding.length);
matrixParameterEncoding[';'] = "%3B";
matrixParameterEncoding['='] = "%3D";
...
}