It turns out the MOK key installed previously was removed during a BIOS update. Upon the next kernel update from Ubuntu I was prompted to install the MOK key, after which OpenCL works even when kernel lockdown is enabled.
It turns out the MOK key installed previously was removed during a BIOS update. Upon the next kernel update from Ubuntu I was prompted to install the MOK key, after which OpenCL works even when kernel lockdown is enabled.