I was also experiencing this problem and fixed it by using the shim bootloader instead of Grub. I'm not sure if using Grub directly _should_ work, maybe there's still a bug here, and I'm also not sure what changed during the Ubuntu upgrade (I assume it was using Grub directly before as well, but never thought to check and it's too late now).
I was also experiencing this problem and fixed it by using the shim bootloader instead of Grub. I'm not sure if using Grub directly _should_ work, maybe there's still a bug here, and I'm also not sure what changed during the Ubuntu upgrade (I assume it was using Grub directly before as well, but never thought to check and it's too late now).