Telerik Reporting supports deep object graphs, i.e. business objects with complex properties and there is no problem to use an expression like = Fields.Activity.Name. Moreover as explained in the Understanding Report Sections
help article, the page sections are processed by the corresponding rendering extension after the report data has been processed. At this moment the report data source is not available anymore and a data source field placed in a page section would display the first value i.e. = First(Fields.Activity.Name).
Having the above in mind, we do not believe the problem is related to your data source or the fact that the textbox is in the page header and there must be something else in your report causing the textbox to multiply. That is why we would appreciate if you prepare a sample project and attach it in a support ticket or provide a link to it here (remember to remove the assemblies).
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