Somewhere between 5.10.12-171 and 5.11.6-174 the kernel and initrd files were moved from /boot to /boot/EFI/com.solus-project.
The links /vmlinuz and /initrd.ing were not updated.
Grub fails to boot in this situation because it cannot find the kernel or initrd files
I repaired it with a custom grub entry, then realized all I needed to do was remake the links
I have two Solus 4.2 installations, one on mbr disk with legacy boot, other on gpt disk with uefi. This only happened to the uefi installation
PS R under Solus is fantastic. Stick with OpenBlas, it speeds R up about 17x.