Parameters Approach To Fill Report Header With Source Data Doesn't Work

It's well known issue, that one can't use any dataset fields in a
report header/footer directly. One of the approach is to create
query-based parameter that basically equals
=First(Fields!@FieldName@.Value, "@DataSetName@") and use that
parameter value instead. But it doesn't work in my case!



My report displays some entity description and is parametrized with
EntityID param. Its header contains entity name that, according to the
approach, is queried from the data source through the EntityName
report parameter. There's important issue: the report is displayed in
ReportViewer control, that is embedded into my application and entity
ID parameter isn't ser by user in ReportViewer parameters area. Its
default value is changed by the application with SetReportParameters()
web method every time a user wants to view the report according to the
entity the user is exploring in the application. But after the report
has been rendered, its header always contains not actual (outdated)
entity name. Nevertheless, the report body contains actual data
(including entity name). If I alter entity ID parameter in ReportViewer
or in web-based Report Manager and refresh report, header displays
correct entity name.



What's wrong in the workflow described?