Bug in query helper?

After upgrading from CI 2.x.x to CI 3.0.0 i've got some errors on my small project and the most of them i could fix, but at this one i'm not sure if it's not a bug.

In one of my models i have the following linde of code:

->join('table', 'table.x= (y - z)')

It is converted to the following sql statement, which is not working:

JOIN `table` ON `table`.`x` = (`y-` `z)`

But it should be converted like that:

JOIN `table` ON `table`.`x` = (`y`-`z`)

So has sth changed here or am i doing it wrong?