Eu tive o mesmo problema que você.
Resolvido alterando o BootOrder usando efibootmgr
no Ubuntu.
Antes
# sudo efibootmgr
BootCurrent: 0000
Timeout: 5 seconds
BootOrder: 0000,0080
Boot0000* ubuntu
Boot0080* Mac OS X
Boot0081* Recovery OS
Boot0082*
BootFFFF*
Alterar pedido
# sudo efibootmgr -o 0080,0000
Depois de
# sudo efibootmgr
BootCurrent: 0080
Timeout: 5 seconds
BootOrder: 0080,0000
Boot0000* ubuntu
Boot0080* Mac OS X
Boot0081* Recovery OS
Boot0082*
BootFFFF*