Run attached app.

You'll see combo box containing three strings.

Click at any item, for example second string.

String will appear at top, but will disappear from list itself, like at picture attached.

After clicking another line, effect will be the same.

After all, combo box will contain only blank lines.

BTW, physically items are at their place and can be chosen.

But they became totally invisible.


UPDATE

If replace line where items added to combo box to simple
///////////////////
cmb.getItems().addAll("Veni","Vidi","Vici");
/////////////////////////

, effect disappears.