CyUSB.sys or CyAPI does not have a limitation like that. What is the device being used?? FX, FX2LP??
Did you by any chance take a CATC trace to see what is happening at the USB bus level?
What is the make of the hubs in question? Have they passed USB compliance? Did your device pass USB compliance?
The devices are FX, the hubs ad the devices are USB compliant, made by System-Group S.p.a.
Honestly, i did not take a CATC trace to see whot's happening: I will look and reply.