BarButton simple tooltip does not show

Dear team,

In ribbon we have a lot of BarButton with only one Label and InutGesture and in most cases we would like to have in tooltip "My label (Ctrl + A)" for instance.

In the documentation for screen tips there is this example which is pretty much what we would like to have:

<!-- Copy (Ctrl+C) -->
<bars:BarButton Label="Copy" Command="ApplicationCommands.Copy" />

But this example does seem to work (even in an extremely simple app with only on ribbon and button).

Thank you for any help.