Random thoughts about the nature of the video/audio:
If Libby was making a snapchat video, then she may not have filmed BG out of concern that they were in danger. If she had been filming Abby on SC crossing the bridge and talking about regular stuff (as I believe was stated at some point), then it makes sense that BG would come into the frame as he approached. The girls talking about regular stuff had to have happened pre-BG clip. Otherwise, I don't think the girls would be talking to each other casually.
Libby could have dropped her phone into her pocket (or to the ground) at the point BG approaches for many reasons... out of fear, to free up her hands, to help Abby finish crossing, to run, etc. etc. Not at all disputing that he was odd or creepy or that they got a bad vibe by the point of the video clip we have seen. It just seems that if she was intentionally videotaping him, she would have had the phone directed toward him (rather than smalltalk with Abby) when he was a bit further back on the bridge? And if LE had that footage, I think they would have shared it because we would all have a better view of his gait (even though he'd be further away, we'd be able to see more than one or two steps).
Maybe she was SC videoing Abby and when Abby asked if he was behind her, Libby did a quick sweep to see that he was indeed behind her and closing fast -- maybe that's when they lose video, but still have a bit more audio (at least enough to hear the "guys" and the "down the hill").
Pure speculation. JMO.