I have noticed this behavior before but never figured out why it happens
I have combo box with 3 columns of widths 4,0,0 Bound Column 1
When the list is dropped down or typed into the entry appears in the box
When the enter key is pressed then the combo appears blank but there is text in it because if you click in the combo or tab to it, it appears highlighted.
If I click off the form to another form and return it appears, but if I try a requery or repaint then it makes no difference.
I have attached a jpg with examples of the 4 stages