In Multi-Step Reports (MSR), calculated fields depend on the underlying data columns being actively retrieved. When you remove or hide the source fields from the Retrieved Fields list, their data is no longer loaded, causing the concatenated calculated field to break.
Resolution:
To hide the individual fields while keeping the concatenated result working:
Do not remove the fields from the report.
Instead, after running the report, use Set Columns → uncheck the columns to hide them from view.
This keeps the fields available to the calculated expression while cleaning up the display and exporting.