The endpoint descriptors are correct.
We have seen this issue ('Insufficient bandwidth') with some of the Host Controllers. Can you test with another PC or reduce the MULT to 2 (i.e. CY_FX_ISO_PKTS =2)
when testing it with ehci controllers I could not see the issue.
testing it on other PC's with xhci the problem remains.
when setting CY_FX_ISO_PKTS to 2 - I can see ISOD2 transfers properly.
I need to use the total throughput of the device so ISOD3 is important to me