Why do the symbols become some unknown words in English?

Hello!
I really want to know why recently when I typed the short form of English words here, it did not show up the symbols? See the below for example:

I'm ( I am )
That's ( that is )
It's ( It is )
Don't ( do not )
doesn't ( does not )

See, all the short form symbols became some unknown words. Is there anyone know why?