Issue with xlsx recipe

[a-pulvirenti_TSGC24]

Hello,

I have an Excel report where I use a data table and in the header I put a placeholder which contains the call to a function which is defined in the helper of the report itself.
Up to version 4.10, that worked perfectly, sinche the function call returned a string, and that string appeared in the table header as expected.
Now I am using versione 4.13 and whenever I try to launch this report, it produces a corrupted file, which Excel has to repair before opening it, while if I try to run the report from the jsReport studio, it does not manage to produce a valid Excel file.
I have tried to act in the template by using xlsxReplace to replace the value on the fly before rendering, but this does not work. It does not work either if I try to put the value in the shared strings replacing the fixed string value I put in the Excel template.

Any clue?

Thanks