To reproduce this issue:
Create a bar chart report.
Select "Assigned to" in the additional group by by clicking the + symbol next to the Group by.
If there are 500 unique users(assigned to), the report will cause performance issue or will be unreadable because there are many variable data points.
Before Instanbul, adding the String field in the Additional Group By would make it available also for Stack by. However, for Istanbul and subsequent product releases, you can no longer bypass the Stack By field filtering via Additional Group By.
Stacking by String is no longer allowed starting in Istanbul because strings, as well as integers or longs, are usually highly variable fields, and very often almost unique. Enabling the seleciton of these types of fields for a Stack By chart generates charts with a large number of points. As a result, viewing these charts from reports, homepages, or dashboard can affect performance.