While creating a report, if a column is used for Grouping and if the 'Auto Expand Groups' is turned off, then the preview/published version of the report will not have an option to expand the columns.
16.3.3
This is working as per the design.
Published reports are considered as a 'Ready to download' copy.
With the amount of data that comes into the report, a dynamic expand can be problematic from a performance perspective.