kyrios I have one 512 Mb partition which i created manually before installing Solus and after installation there was no entry in BIOS. I did not forget to add boot and esp flags to it either. Bootloader is in that partition, i can mount that partition and i can see the files but the entry in BIOS was just not created automatically. I do dualboot, but both OS'es are on separate drives, and Windows drive was removed during Linux installation to not mess anything up.
And i notice that if i touch my drive configuration somehow (remove one drive, boot when there are external drives connected and etc.), boot loader entry for Linux in BIOS goes away again, i need to boot solus usb and do the same efibootmgr command once again, only then i can boot again as normal. I have no idea why it's not sticking there.
I did have to do something like that a year ago for my Solus Mate installation, but back then, it installed well, bootloader was working fine, and just after getting both drives back to the computer, boot loader entry was gone. When recreated with efibootmgr it stayed in BIOS all year long without any problems.