SYMPTOM:
When more than 255 columns are added in a single report, the following message will be displayed in Strategy Developer 2021:

'The number of columns in the report (including headers and data) exceeds the limit of 255. Only the first 255 columns will be displayed'
CAUSE:
There is a limit of 255 columns to be displayed for a single report and any columns exceeding 255 will not be displayed.
ACTION:
It is possible to run a report exceeding 255 columns, but only the first 255 will be displayed. Please contact Strategy Technical Support for the latest status of this issue.
WORKAROUND:
Split the report with more than 255 columns in smaller reports.