Not only --from=si is broken. Also --to=si is broken:

$ numfmt --to=si 3000
3,0K

In order to not break backwards compatibility, you probably have to introduce a switch --lowercase-kilo such that --to=si produces proper SI compliant output. Then have --from=si accept both uppercase and lowercase k.

I have to say, that uppercause K is quite common, but it is not correct as far as SI prefixes are concerned.

Also note that Ki in iec-i mode is quite correct. I'm torn about iec mode. I believe that people silently switch 1000 for 1024 and use the lower case k as well as uppercase K. Maybe numfmt should have an option to accept/produce both here as well?