xp:checkBox in edit mode does not match checked value from bean

on an xpage I have checkbox control with data-binding to a bean e.g.:

In the back-end I load the value from a Notes doc. When I load the xpage I get the correct value, but when I edit the object the value turns to false. It seems that it cannot match the value from the bean ("YES") with the checkedValue.

Is there a bug and a workaround r should I switch to a checkboxgroup control?