My first thought before seeing the video is that as you are splitting the signal, you are also splitting the voltage so that may possibly be the issue, but the video shows it working with three Volcas, although not all three running at the same time. I thought maybe an answer may be in the comment and found someone mentioning the same concern I had:
The only problem with this solution is that you are splitting the voltage between all three jacks. With only 3 of them it seems the devices are still able to interpret the digital signal, despite the voltage being dropped, but there is a chance that with 4 or 5 devices they would begin having trouble identifying the peaks and troughs of the digital pulses.
This can explain why my Volca Bass goes nuts when using this splitter.
Something I noticed is that they are using mains power adaptors for the demo for the Volcas, which may make a difference.
A little more sifting through the comments, I found this:
seems to work with 2 volcas but not 3 . The 3rd gets jumbled notes 
And this:
i tried the exact same setup (also with tr 8 instead of beats) , 2 arturia, 1 novation cable, but if i have 3 of them connected sequencer 2 doesnt sends anything. any combination of 2 works fine tho. any tips?
I think that the signal becomes too weak trying to split it three ways, corrupting one of the streams. I use a Belkin headphones splitter exactly the same as the one in the video for splitting CV and Gate signals and although Gate doesn't suffer as bad from the exercise, CV certainly does get divided with each split producing a lower voltage (and no doubt current), making a split CV signal unusable for accurate pitch tracking (but a bit of fun for modulation routing).
