cubfan's bubble based farm not working.

I built cubfan's bubble based guardian farm on my hardcore server. But it's much more ineffective in this snapshot. I remember building it on my last server which was 1.15.2 and it worked perfectly. Now, for some reason, 1 of 10 guardians gets to live and the others die in the lava.
https://www.youtube.com/watch?v=Q1U5FWYBr80