i Have a similar problem with comboboxes in our application. I am using QT 5.15 under Win10.
Our comboboxes are not editable, but if all text in the editfield is marked selected, and it is the text of an item with an index greater 0, the return-key selects the first item (instead of calling the default-pushbutton in modal dialogs f.e.). This was not the case with QT 5.7