The Preview/Post button used to present a preview of a post with a capability to edit it. It now posts without the preview. What happened?
We change that months ago. You can just edit your post if you made a mistake so that would make a preview redundant.
The preview was especially helpful when things like Bold, Underline, etc. were used. In any case, if there's no preview why have Preview on the action button?
You can change that in your profile to preview the post again.