So chatting with @Joe-Gill We narrowed down what the problem.
First, the backstory.
So Multicast tasks would work for some machines, but not all. And, to add to that, it was always the same machines that failed to work. If the machines that failed were subsequently put into their own group, they would multicast without issue.
This lead me to ask if the machines that would fail were on a different switch from the rest of the machines, and indeed @Joe-Gill found out that this was the case. They are using Meraki switches and they replaced a few of them. Those that were replaced do not work fluidly with the older switches in place.
@Joe-Gill keep me truthful here, but this is the gist of this information.