Disappearance combobox Windows 2008/2003 using WIN32_NATIVECTLS

I encountered this problem enabling WIN32_NATIVECTLS on combo-box

Combobox that expands upward

I select an item and ... combobox disappear.

When i push "tab" the combobox appears again.

The problem occurs on Windows Server 2008,  Windows Server 2003, with runtime 9.0.1 e 9.1.2 (I didn't checked older runtime . . .)

I solved with workaround: after nft-selchange I mage the combo invisible and after visible.

           evaluate event-type

               when ntf-selchange
                    inquire cbox-pag, value in w-combo-pag
                    move w-combo-pag(1:3) to w-codice-pag
                    perform leggo-pag     thru leggo-pag-ex
                    perform ab-situazione thru ab-situazione-ex
                    modify cbox-pag, visible we-off
                    modify cbox-pag, visible we-on

It might be a runtime bug ?

Any suggestions?