the most reason is :
Ubuntu stays with version 2.04 Grub that do not support multi-files on boot ,
since start grub Manjaro , we boot with microcode & kernel , not just kernel
also Ubuntu require a shim/key from micro$oft that has been change 2 times , and give now lot of headhache with all UEFI version with SecureBoot , because they have to refuse older keys/shim( included micro$oft versions ) .
this happed 2 times boothole 2020 and 2021 grub
Ubuntu do not use last version release , because it include multi-files boot patch
and there are theses bugs
https://savannah.gnu.org/bugs/?61622
https://savannah.gnu.org/bugs/?60458
https://savannah.gnu.org/bugs/?61233
see this and date about version 2.04 ( big changes on march 2021 )
https://git.savannah.gnu.org/cgit/grub.git/log/?h=grub-2.06
also for any others Distribution DE , they have more than 100 - 200 patches that they cant now revalidate