Query Builder is not null

I'm in the process of upgrading my project from 3.0.0 to 3.1.8 (in one go).

After going over all the required changes listed in the docs, I started testing my project, and realized something is different in the way CI is treating IS NOT NULL statements in the Query Builder.

In my 3.0.0 project, I used syntax like this:

PHP Code:

$this->db->where(array('book_code'=>$book_code,'poi_num is not' => null)); 


to produce
WHERE book_code=[code value] AND poi_num IS NOT NULL

but now, in 3.1.8, the same code produces:
WHERE book_code=[code value] AND poi_num IS NOT

So I changed my Query Builder syntax to:

PHP Code:

$this->db->where(array('book_code'=>$book_code,'poi_num is not NULL' => null)); 


And now it works fine.

But:
1. Is this the correct syntax to use in such cases?

2. Was this a known issue in one of the upgrades between 3.0.0 and 3.1.8? If so, where was it documented? I'm worried that if I missed this, I might have missed other important changes.