see the VMedia link I posted earlier:
VMedia Internet Traffic Management assigns the highest priority to time-sensitive and priority Internet traffic such as TV and VOIP, a high priority to applications such as web browsing, email, instant messaging and streaming, and normal priority to other activities.
Netflix streaming is lumped in with web browsing and that category is probably allocated just enough bandwidth for HD quality during congested periods. Netflix recommends 25 Mbps for 4K streaming.
I believe third-party ISPs contract with each incumbent for a specified amount of total bandwidth and it's possible VMedia may not have enough capacity with some ISPs in some locations. That would explain why some end users have problems and others don't.