The attached file ---bare html, no header or trailer-- passes the "Validate Current Document" and so presumably contains completely valid HTML. Besides that, it's really simple and short, a cut-down of a somewhat longer file I made for posting about this issue.
I just restarted my Mac, opened DW, opened this file for edit in Design View, selected the final word in the second paragraph ("beyond") and deleted it. The entire second paragraph disappeared. I immediately saved to another file, re-opened it, and confirmed that the 2nd paragraph is, in fact, missing from the file.
Using this code, do you reproduce this issue?
Have you seen this kind of issue before?
Note: Yes, editing in Code View sidesteps this issue but that's an emergency workaround, not an acceptable alternative for production work.