Can you try the following:
sudo efibootmgr -c -d <disk> -p <partition> -l "\EFI\systemd\systemd-bootx64.efi" -L "Solus Boot Manager"
On my PC I had the same issue; The Solus installation did create the boot partition, but it wasn't added to the BIOS entry list. If this is also the case for you, this may set the boot entry for Solus as it should have done.
Note: After first succesful boot, Solus itself may add the additional 'UEFI OS' default boot entry.
Just in case: Make sure that you enter the disk and partition of the boot loader and not the root partition.