In my opinion, this is just how Indesign works. The keep-options are meant to work in threaded textframes to keep flowing text organized. In your case, you start a new textframe at the bottom of the page and then move on to the next page. The question is more “Why would you do that?”.

If you can open the file, is it possible,to split it. Removing first and then second half of the pages. If doing this, work on a copy to start.

Since it is only a 12-page document it will take not too long to do it manually. I would say that is the cost of not using paragraph styles ?
The easiest way is to find/replace by text specifications like font, height, or color and then change to a paragraph style, but since they are not into paragraph styles…

James, you can toggle this functionality on and off in the preference under “file handling”. Works for me in InDesign CC2019