I haven't got any response to my question so I will try again.
To me it seems like there is a bug in Qlik Sense (both previous and latest version) as it is not possible to select a dimension value from a stacked bar chart by simply clicking on it.
When I click on a dimension value - e.g. CAN as below the only thing that happens is that the chart is activated:
However, when I do the same thing in a non-stacked bar chart I get the expected outcome - i.e. the value CAN is selected:
I get that you can select by highlighting a range as shown below but the most intuitive is to click on dimension value as you can do in all other charts.