Steps to reproduce the issue

Joomla 3.9rc1

Think like a regular everyday Joomla super admin (and not like a developer/contributor)

in Action logs admin, a normal Joomla super admin will assume the search box allows them to - erm - actually search for something...

ok, so I wanted to search for "logged in" to see what users have logged in today

Expected result

List of actions with "logged in" in their Action text.

Actual result

No results

The actual sql run is only searching on usernames

SELECT a.*, u.name
  FROM ltm4b_action_logs AS a
  INNER JOIN ltm4b_users AS u 
  ON a.user_id = u.id
  WHERE (`u`.`username` LIKE '%logged%')
  ORDER BY a.id DESC

in which case, the search is doing exactly the same as the user dropdown and is therefore useless for searching anything other than usernames (and even that fact is not communicated)