MySQL – Why UPDATE Does Not Use INDEX but SELECT Does

I'm actually struggling understanding the problem here. I read everywhere saying that UPDATE benefits from indexes on the WHERE clause.

However, this query,

UPDATE `documents` SET `read`="1" WHERE `docid` IN (<subquery>)

does not seem to be using an index.
The table documents has an index on docid and on read.

When I do an EXPLAIN, I see possible_keys = NULL and rows = 8011008 (full table). The subquery does use keys and read the proper rows (2 rows).

In the other hand, this query:

SELECT * FROM `documents` WHERE `docid` IN (<subquery>)

does use the index on docid and runs very quickly. It reads a few more rows than needed (as per EXPLAIN), but totally acceptable.

Is there any explanation for this?
I use MariaDB 10.

As a funny note (regarding SELECT), if in the <subquery> I use a UNION, while the subquery seems to get the proper number of rows, it seems the primary query does NOT use the index and does a full table scan.

If in the UPDATE I use JOIN instead of IN, the indexes are properly used. I resolved my problems by using JOIN.