Wikimedia Bugzilla is closed!

If a page is moved, the edit summary for moving it is only added to the history
of the "source" page. A log entry is made at the "destination" page without the
edit summary. For instance, page A is moved to page B with summary 1.

Now, page A's history shows:  A moved to page B, summary 1
and page B's history shows:  A moved to page B


If the page is then moved back, the log entry for the original move disappears.
Thus if page B is now moved back to page A with summary 2, the result is

page A's history shows:  A moved to page B
page A's history shows:  B moved to page A
page B's history shows:  B moved to page A, summary 2)


This means that the page is now at A, but from its history, neither summary 1
nor 2 is visible. Summary 1 has apparently disappeared entirely.