Here's the outcome of the ticket with Qlik Support:

R&D has investigated the issue and come to the below conclusion:

"For a stack bar chart, there is a case when we have both positive bars and negative bars. So the tooltip would show both sum of positive bars and sum of negative bars.

The problem here is the words "positive" and "negative" are also used as values of Dim1 field.

If "total" is used as a value of Dim1 field then we will have the same problem."

R&D has declared this issue to be working as designed, so there are no other suggestions. If you have any questions, please let me know.