This is because you've completely changed the level of detail in the view by adding this dimension. It is working the same way only for each distinct dimension value and they are being overlayed on top of each other. If you place the Incumbents group on rows you will see what I mean. All those rows are overlayed on top of each other.
The question is how do you want to represent this waterfall with color? Can you mock up a block schematic with values showing what you want? It doesn't even have to be in Tableau just a waterfall diagram with labels and values for each of the mock color segments.
First add the field into colour then change your table calc to be based on the below specific values:
Create a copy of this this field and create a dual axis:
On the second marks card remove everything in the marks card and colour the gantt as white. Then add in the label:
Finally you get this:
Thanks and please see the attached workbook.
Waterfall_v10.2.twbx 87.0 KB
Nicely done Mavis!