It was a server-side block, from Cloudflare (security rule specifically). I’m very familiar with it, having used the same service over a decade. They are able to tweak the overall security level, or specific WAF rules for the endpoint in Cloudflare. They also have analytics that will show them exactly how many cancellation requests would be blocked. The fact that they totally ignored these details in my ticket, is concerning.
I picked up a 4070 and have been really happy with the low power draw (and therefore heat) after undervolting it.