I think, that the behaviour has been changed in the 8.6.2 version of WBuilder/Pro.
If one uses an instance of the WbObjectComboBox one can say, that the box is in "editable" mode. That means, that the user can select an entry in the box, but is able to change/edit the value in the text field after that.
Now you can not edit the stuff any more - try to use the backspace key and you might notice, that the the cursor does not move any more - seems to be that you are only allowed to edit the text field to a values known in the box.
Use the WbObjectComboBoxExample and change the personList instance of WbObjectComboBox and check it out ...
Ok, sorry for this ... seems to work as programmed. If you have a text in the entry field and the cursor is at the end of this text field you might type the backspace key, but then the system tries to expand the rest to the original value and then nothing gets deleted.
If you type in the delete key (cursor is positioned somewhere in the text) you might be able to delete the character at the cursor position - BUT do not delete the last character - otherwise you get the original contant again.
To change the content: type in the new value again from the begin of that text field. If the new string is shorter than the original text just type the white space key. Then the rest is deleted.
In your program make a trimBlanks of the content of the widget value.
The behaviour is located in WbObjectComboBox>>cellValueChangedCallback: anEvent