I'm using Excel 2019 on Mac (v16.63.1), and since last week I'm experiencing this problem: when exporting a sheet as a PDF and selecting the option "Best for electronic distribution/accessibility (uses Microsoft Corporations' online services)", the decimal commas are automatically replaced with points. This doesn't happen when I manually change the option for "Best for printing".
I checked the language and settings of my computer (French [Canada] + uses the comma as a decimal separator), I also uninstalled and reinstalled Office, and the problem remains.
Since the problem occurs when selecting the option "Best for electronic distribution/accessibility (uses Microsoft Corporations' online services)", I'm wondering if it can be a problem on your side.
The Tech Community isn't a support site so I think you may have landed here/been redirected here in error. The best place for an investigation like this is going to be answers.microsoft.com. If you click here I have crafted the URL to place your question in what I suspect is the most appropriate part of the forum to get you the support you need.