SO question about disappearing row

Saw the following question on Stack Overflow: http://stackoverflow.com/questions/39550510/select-for-update-returns-zero-rows

One transaction updates a row with the same values, causing another transaction to return an empty set. This is surprising as the row passes the where clause both before and after the update.

Is there a way to have Postgres explain why the outer query drops a row even though the query plan does not show a filter?

Cheers,
Andomar