So it's not Windows 11 after all
.
I suspected it was the 1.54 firmware in the BDR-211UBK being extra picky about the USB-SATA chipset, so I got the Benfei USB-SATA adapter recommended on the MakeMKV forum, and the BDR-211UBK works perfectly with it.
So this confirms to me that the BDR-211UBK on official 1.54 firmware is a lot more picky about USB-SATA chipsets than the BDR-212UBK on the official 1.05 firmware. Working great on Windows 11.