I can't find a way to do this. I think that there must be a rule that if the label is bigger than the bar, it will not be displayed. It would obviously get very messy if you had multiple such bars within the same stack.
I can only suggest using a different visual (side by side bar) or providing a tabulation alongside the stacked bar plot for people to refer to for the figures. You should know that you can also hover on the chart to get the values.