I am not sure if this is issue is how Android Oreo is intended to work or if the feature is broken on the Nokia 8. The fact this this issue occurs intermittently suggest that it is a bug.
I have found that the multi window (split screen) feature regularly activates when I don't want it to.
My understanding is that the feature should active (and deactivate) when holding down the square (recent apps) button.
In practice, if I have an app open, press the centre (home) button to return to my home screen, and then select another app from the home screen, it will occasionally open the new app and the one I was just using in multi window mode.
I find this issue extremely frustrating, as I then have to either drag the bar between the two windows to close one, or hold down the square button.
Please Nokia can you fix this, or a least add the option to disable the multi window feature all together.