had the same issue with blocksize and was able to resolve it. Maybe it helps you:
The issue was that the HPE installation package for the tape library / LTO drives (cp032931) is also installing the driver for the SAS controller (HP P411 in my case).
But: this is a special "media driver" (you can see it in the device manager), looks like it's limited to 64k blocksize.
I installed the real P411 driver afterwards (cp028045), which replaced the media driver, and after a reboot Veeam was able to use the tapes again (512k).