As per the request in the poll, I hereby share that I was impacted by the grub_is_using_legacy_shim_lock_protocol issue. I was able to fix it using the solution here: Get a black screen with “Error: grub_is_using_legacy_shim_lock_protocol not found" - #6 by Aragorn
I do not understand however why this update uses a “let it break and then tell people how to fix” approach.
In addition, manjaro-kde-25.0.7-minimal-250812-linux612.iso gave me failed to start user login management and didn’t start so I had to use an older iso I thankfully had lying around.