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To replicate the issue, see this example: https://stackblitz.com/edit/mtehnv?file=index.ts. When creating the Excel export file and opening it, Excel complains "We found a problem with some content 'Export
The problem appears when the data contains a character called "Substitute character" (character code 26) (https://en.wikipedia.org/wiki/Substitute_character). That is a control character which is not a written symbol, but it's still a valid character for example in the Unicode. Excel files can also contain that character, so it seems doable to generate a valid Excel file containing it.
(It seems that Chrome shows that character is a box, but Firefox doesn't show it at all.)