This has been happening for the last few betas of ios11 (iPhone/iPad) and continues with the GM which came out yesterday, so I dont think it will be fixed from Apples side. I think it could be an issue with AtariAges forum software.
Im a pretty careful mobile typist but have taken tonavoiding contractions like wont cant Im and isnt here. They show up when composing a message, but disappear when posted. Very weird and seems isolated to this site as I havent seen it anywhere else.
iOS is released for all users soon so it would be great to get ahead of this.