"Submit a Story" Question

I've been working on a post that includes Markdown and HTML. Is it normal after switching back and forth between Markdown and HTML, some HTML tags are removed such as the "center" tag? I'm saving my HTML in Word because the editor keeps removing my HTML when I switch to Markdown mode.

Also, yesterday I didn't finish editing and today after I logged in, all my data was missing. Is that common? Fortunately, I have saved my work in Word.