So I've been having the same ongoing highlighting/text selection problem that others have had. I do NOT have Smart Text Selection on. In fact, when I turned it on to try to see if that alleviated the problem, it did not. I never post on the forums and I've never done a screencast before, but this problem is to the point where I can't use Logos without it occurring regularly and disrupting my work. I even lost a note trying to undo it the first time it happened--very frustrating.
I'm not sure if it never happens when the interlinear is turned off because I almost always have it on, but testing it for the screencast I could not get it to do the bug when the interlinear was off. As others have said, when you select a word/phrase, the ending point then becomes the starting point for the highlight/text selection which then goes for many paragraphs.
Here are two short screencasts:
http://screencast-o-matic.com/u/n5g5/logos6-highlighting-bug-2
http://screencast-o-matic.com/u/n5g5/logos6-highlighting-bug-3
Hopefully this can be quickly fixed. Thanks!