Currently, the empty cells in the exported Excel file contain zero-length strings. As a result, the ISBLANK() function returns false for them while the cells essentially do not have content.
Please allow the empty Grid cells to be treated as blank in the exported Excel file.
For the time being, you may extend the formula to also check whether the length of the cell content is 0. For that purpose, you may use the LEN() function as suggested here: https://learn.microsoft.com/en-us/office/troubleshoot/excel/isblank-function-return-false#workaround.