I try to reproduce the issue in my test environment. However, when I display the report data with Matrix and put the string data type values in the detail cell. The report render the first value of each column group in the datail row.
What's the version of the SSRS instance you using? Did you designe the report with Matrix control?
Please post a screen shot of the report designe structure and the dataset with sample data to us by the following E-mail address?It is benefit for us to do further analysis. E-mail: firstname.lastname@example.org