Just some random thoughts, since we're talking about proxies.
If someone on the same range is abusing, you will get into trouble along as well, even though you have different sub net and not spamming as hardcore as the other.
Lets say you have 192.168.0.1 and another guy is 192.168.0.2 abusing, you are his next door neighbor, high chance you'll be on the list.
So for instance, these guys like proxymillion restrict the ip to platform, and you think that's cool, you're the only spammer using these cool "non /24" ips, and then bam, they got banned.
Logically, proxy sellers have all /24 in many subnets and they allocate to you these cool shiz subnet that isn't /24, but guess what? They also allocate these same cool shiz non /24 to 1000+ other spammers with 1000+ accounts.
They have finite amount of proxies in finite amount of subnets. In the beginning of offering the service they may have clean ips, but sooner or later it gets darker but they don't buy new ips.
Furthermore, once someone gets banned, they rotate the IP, and find out it gets banned as well. The seller can either give you lesser abused IPs, or they can give you IPs that someone rotated like an hour ago that's been abused.
Does that mean the seller is out of business? Nope. They just re-allocate these abused IPs to the next "victim".
I am not sure how often they perform a subnet ban sweep, but just a thought.