Does anyone have experience debugging CMSIS-DAP firmware itself?

[ataradov]

There are no ZLPs in HID. All HID data is sent as complete fixed size reports at least in case of CMSIS-DAP implementation. All FS implementations declare 64-byte reports in the report descriptor.

Plus that was changed a long time ago and report size is now automatically detected on all platforms. And no matter what you have to send the amount of bytes in the report. And 512 was a hack specifically for a MAC before USB handling was updated on that OS.