com.sendgrid.TrackingSettings use @JsonInclude(Include.NON_DEFAULT)
so when you set
OpenTrackingSetting setting1 = new OpenTrackingSetting();
setting1.setEnable(false);
settings.setOpenTrackingSetting(setting1);
ClickTrackingSetting setting2 = new ClickTrackingSetting();
setting2.setEnable(false);

the jackson will give a empty jsonobject, and the http request will get response 400