FULL PRODUCT VERSION :
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

A DESCRIPTION OF THE PROBLEM :
It's common when creating a ComboBox of user-defined types to set a custom cellFactory which overrides the updateItem method of ListCell. This allows you to define custom logic for transforming objects of type T into the String representation which is displayed in the Cell.

For example:

            // Customize the appearance of the cell.
            override def updateItem(item: MyType, empty: Boolean): Unit = {
                super.updateItem(item, empty)

                if (empty || item == null) {
                    setText(null)
                    setGraphic(null)
                } else {
                    setText(item.printableName)
                }
            }

However, when the ComboBox is initially rendered, the "button cell" that it draws to occupy the space of the control does not use the user-defined cellFactory. Indeed, from com\sun\javafx\scene\control\skin\ComboBoxListViewSkin.java:

        buttonCell = comboBox.getButtonCell() != null ?
                comboBox.getButtonCell() : getDefaultCellFactory().call(listView);

Instead of using getDefaultCellFactory, this code should call comboBox.getCellFactory() and use that to construct the button cell.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a ComboBox with a custom cellFactory which overrides updateItem.

Render the ComboBox.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The custom-formatted item should appear in the button cell
ACTUAL -
The default-formatted item appears in the button cell

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Currently you can manually call ComboBox.setButtonCell with the output of your custom cellFactory.