When enabling Total Labels in Stacked Bar Chart visuals, the labels will show the correct data format for most bars but will sometimes lose it's formatting. It seems to be when a bar doesn't have more than 1 stack. Example in screenshot below:
Does this problem occur in Power BI Service or Power BI Desktop? If it works fine in Power BI Desktop but shows up as an exception in Power BI Service, this is a known issue and engineers are working hard to fix it.