I don't know if this the same issue, but it happens constantly to me when i reinstall my system. Sure, i dualboot with Windows, but i recently cleaned both disks and just for experimenting installed just Solus on my main drive leaving another disk completely empty, boot entry was created. Then i tried reinstalling Solus the same way, (cleaned drive with gparted before that) and this time there was no boot entry.
In those cases when boot entry is not created, you can easily recreate it with efibootmgr. You can use your solus install usb, if so you will need to install it first (sudo eopkg install efibootmgr
) or just use efibootmgr from your Ubuntu. Use command sudo efibootmgr -c -d /dev/sda -p 1 -l "\EFI\systemd\systemd-bootx64.efi" -L "Linux Boot Manager"
, you also can read about this application and its use on Arch wiki (https://wiki.archlinux.org/index.php/EFISTUB#efibootmgr). You can probably change the name from Linux Boot Manager to something else, if it's already used. Just make sure to check in which disk solus and its boot partition resides and change the command accordingly. Just restart after (i would restart with my usb inside the port to not mess boot entries then remove it if everything works) and check in bios, boot entry should be there.