From T56721 comment 2:

When I copy a portion of content beginning with a header, then paste it
elsewhere in the same document, everything is retained /except/ the header.
However, if I begin before the header (the paragraph before the header, for
example) the header is retained.

For example, copy everything from 'Start' to 'Finish' on this test page:
https://www.mediawiki.org/wiki/VisualEditor:Test1234567

Everything is retained except the H2 formatting on 'Start'. See this
screenshot as an example too:
http://images.wikia.com/trevortest/images/5/54/Header_is_lost.png

Version: unspecified
Severity: normal