Not necessarily with Mikrotik hardware/software, but USB3 devices are often tricky, and USB hubs (even if in your case represent a woekaround for he “main” issue) are often a complication.
Everything is possible in these cases, as an example there are several reports of issues with some SanDIsk USB sticks:
http://forum.mikrotik.com/t/rb5009-routeros-7-9-1-usb-slot-keeps-changing/167185/1
http://forum.mikrotik.com/t/usb-disk-issue-after-hardware-reboot/172598/1
it could be the same or a similar (but different) issue with the specific controller in your NvMe case/converter.
Since the USB3 speed is unavailable anyway, due to the hub, it would make sense IMHO to try an older (slower) device or another case/converter/device.
About the flag value, you can try something like:
:put "$[:tostr [/disk print as-value]]
see:
http://forum.mikrotik.com/t/how-to-use-the-output-of-print-in-scripts/152280/1