When ran the attached test case with JDK 8u161, 8u172, text is wrapped but when ran with JDK 9.0.4, 10+43, 11-ea+3, text is truncated with ellipsis.
This does not happen in macOS.

 If i use alert.getDialogPane().setContent( new Text(""));, it makes the window very wide to fit the text on a single line and no wrapping.

WorkAround :
alert.getDialogPane().setMinHeight(Region.USE_PREF_SIZE);