Packagist (composer) is newer, as it is automatically updated from github, and therefore also contains hotfix releases. The zip on the website is only updated wth official releases, so it is still at 1.8.0.
I am not sure what you mean with "master" and "slave".
Fuel uses a readwrite connection (which is the default) and a readonly connection, which can be configured. Which of the two is used in the Database_Connection instance depends on the parameters passed when creating the instance ($writable must be set to false, and the db definition must define a readonly database name).
So if something doesn't work the way you want it to work, start by showing us what your database conifugration is, and what code you are using to identify the fault.
I've checked the ORM, if it has separate read and write connections defined, it will explicitly use them by passing the connection to execute() when running the generated query.
If not, it will use the mechanism used in the database layer. It seems the problem is that the static frontend to the database layer, DB, doesn't use the writable flag when it creates a Database_Connection instance, which if so, the test in my previous post will prove.
If you can confirm that passing (null, null, false) fixes your problem, I can push a fix to the repo.
Before, I used fuelphp 1.7 for my project without master/slave configuration. When I upgrade to fuelphp 1.8, I use master/slave configuration, so I cannot modify all source code to use: