Steps to replicate the issue: (try this on a test Wiki)

• Edit a section and change some text
• Then click on "Edit" or "Edit source" in the main menu
• Then save the updated text

What happens?:

Only the section text is saved; the rest of the article is silently lost. No warning was issued that the "section" editing URL parameter was silently lost... or that information was going to be lost anyway.

What should have happened instead?:

The section should have been saved, together with the rest of the article.

Origin of the bug: the "section" URL parameter should have been kept during the editing process.