Lenovo ThinkCentre M73 - "secure boot" issue

I’ve just installed Rocky 8.4 on my Lenovo ThinkCentre M73 using a USB stick created on my MacBook by dd’ing the Rocky-8.4-x86_64-dvd1.iso image onto /dev/rdiskX - usual sort of thing I’ve done for CentOS many times before. However, when I booted from the stick I got a warning “Secure Boot Violation. Invalid signature detected. Check Secure Boot Policy in Setup” and startup would go no further until I’d gone into the BIOS and disabled Secure Boot.

Having completed the installation (total breeze - thanks, guys!) I re-enabled Secure Boot before booting off the hard drive, but again I got the “Secure Boot Violation” warning so I had to turn it off again.

In the BIOS Security settings it says “Secure Boot … will prevent un-authorised OS be loaded”, so why is CentOS 7.9 2009 authorised and Rocky 8.4 isn’t?