Wilfried, I think you have misunderstood what marking a button as the Close button does. This setting does NOT mean that clicking the button will close the form.
Defining one of the buttons as the Close button defines what action to take when the form window is closed manually, for example by clicking the 'X' button in the top-right corner. When you close the window this way, an action will be taken corresponding to that of the Close button. So you could make the Save button the Close button, and your changes should be automatically saved when you click the 'X' to close the form, or more usually, you make the Cancel button the close button, which means that closing the form abandons any changes.
George, the setting on the data view in the form that controls whether the form closes when you click the Save button, or not, is 'Close on save/cancel' which you will see in the properties panel when you have the dataview selected.
However, in my apps I usually replace the standard Save and Cancel buttons with action buttons with microflows that perform these actions to give greater control. For example, the Cancel action microflow would rollback changes then close the form. The Save/Close action microflow would commit the changes and close the form, but I might also add a Save button where the action microflow simply commits the changes, but does not include the close window action.
You probably have setup the save button as the close button of a page. Edit the button properties and check the Is close button option.
Thanks a lot for your response. I have checked the setting and it was set to 'No'. I'll just assume it is a bug and proceed with the training. I can send you my code directory if you want to see the behaviour.