I've done some testing and can reproduce it now with the infinite scroll feature enabled.

In the last paragraph of https://www.flyertalk.com/forum/31351272-post21.html there is an instance of "don't" which will render correctly when using that direct link.

The apostrophe is also correctly rendered if you access via AC Drops Domestic flight @ Canadian Customs and then scroll down to post 21

If however you have infinite scroll enabled and start at AC Drops Domestic flight @ Canadian Customs and then scroll down to trigger the loading of page 2 posts when you get to post 21 there will be a space instead of the apostrophe in Chrome and it will be stripped in Firefox (both on a Mac). I checked the page source in Chrome and it's just a blank space, there is no control or non-printable character in the text.