Unexpected update behaviour

Greetings.

I've hit a strange issue. This is a simplified setup.
First, run create.sql to get a couple of tables.
Then, run the update query.

Tested on 9.2.6 and 9.3.2.

Now:
- if I remove the UNION ALL part of the inner query, UPDATE works;
- if I move the `raw` subquery into the CTE, UPDATE works (but takes
hours on my full data);
- if I convert this UPDATE into a SELECT, I get the row to be updated.

Could you kindly explain me why the query as it is updates no records?

--
Victor Y. Yegorov