Sorry to carry on but Lin, I think the simplest answer to you question is that it happens because something is not quite right here.

The same has happened to me and others here. I had one the other day when I tried to post a Wordle share. Everything looked fine on preview but the characters for the coloured bars turned into question marks when I posted. My previous post attempted to come up with a possible reason why this might be:

A preview displays the message as it is interpreted from the message submitted in the input form. When you make a post, this is added to a database and is retrieved from there to, eg. display a thread. If there is a discrepancy between the character encoding used by the database and the one for the input, we could wind up with characters getting lost or distorted.

Anyway, I think further debate would need to go to a tech thread.