Seems related to this:
*.csv files (comma-separated) used to open as tables in Excel (automatically) but then started opening just as plain text, not tables. (and now Excel expect semicolon instead of commas -- then opens as table.)
Perhaps relates for many Excel versions, but for me seems this happened after recent upgrade of Excel in Office365 on OSX - to Excel 16.31 19111002
On Windows there is explicit "List separator" setting in the Region / Language:
but this one seems hidden somewhere in MacOS / OSX (i.e. there is not explicit "List separator" setting (or some similar parameter)). This seems to be set in OSX when you're changing your REGION / COUNTRY. (Apparently Excel expects that in German-speaking countries people use semi-colon instead of comma in *.CSV files: http://forum.provue.com/t/using-semicolons-instead-of-commas-in-european-format-csv-files/2780/11)
It does not help to just changing settings like "Number separators - Grouping / Decimal" settings in System Preferences > Language & Region > Advanced ..
Changed Region to UK/US, but then tweaked all settings back to normal:
1. System Preferences > Language & Region > set to "UK" (or US)
2. Customize System Preferences > Language & Region > Advanced.. -- changing there Number / Decimal / Currency / Measurement units to the ones you have in your real country.