Are these subdomain properties on different A class ip blocks or different C class ip blocks?

It think this all depends. If the IP addresses are in the same neighborhood or on the same subnets as each other then I would say yes. But beyond that you have to think about several other foot prints to look for:

• Are the nameservers the same?
• Are these ip addresses assigned to different regions?
• Are you interlinking these web properties?
• Even the fact that the subdomain is still associated with the domain makes nervous and only because that is easy for Google to track. If you think about how may other data points they use to find footprints in their algorithm, I don't see why that wouldn't be one of them.

I would be careful with RankBrain continuously evolving and seeing how much turbulence there has been in the serps lately. Personally, my small PBN is completely on separate A Class IPs, with custom name servers, different hosts, and I only put premium content on it. It's not great for quick affiliate gigs, but it certainly helps sustain long term growth.