Yes, I have seen that video. As I mentioned in the comment you’re replying to, that video was linked to another protest unrelated to BLM that happened somewhere in 2014, only people started recirculating it as if it was a response to the Dallas shooting and happened during more recent protests. It’s spread around deliberately to smear BLM and discredit their movement. It’s design is to take the focus away from their true message, which is simply to stop brutality and racism against blacks.
You can find footage from it here: https://www.youtube.com/watch?v=D3-uEQdnchI
That’s a video dated to 2014. It should also be noted, that while they intersperse the video with footage of large groups of protests, the footage in which the chants happen only shows a smaller group of an estimated 200 protesters that likely moved away from the organized peaceful protest that involved tens of thousands. This was likely done to give the false impression that the full protest was in on the chants as well.
If it did happen during a BLM protest though, I’m sure they would be quick to disavow those protesters as they have disavowed anyone that displayed violence during any of their protests.