I'd assume that a text formatted cell shouldn't be doing that, yet it was.
I see, then my first reply was premature, because I had not seen it doing this all my life. I tried it today with Excel 2003, 2007, 2010, 2013, 2016. In no instance did it do what your wrote, i.e., remove the comma in a text-formatted box, and the default autocorrect rules don't have any rule that looks like they would do this, either. So I guess maybe you experienced a bug that has been fixed, your had some weird custom autocorrect rule enabled, or it may have had something to do with unusual region settings (which of course affect how Excel treats dot, comma, and semicolon - and over time I saw a bunch of bugs which only happen for some region settings).