I believe I've fixed this with the following code added to JCustomPanel.saveUI()
Component focusOwner = FocusManager.getCurrentManager().getFocusOwner();
if (focusOwner instanceof JFormattedTextField) {
JFormattedTextField tf = (JFormattedTextField)focusOwner;
try {
tf.commitEdit();
} catch (ParseException ex) {
}
}
Do a core update to try it out!