As a workaround, a "bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi" in an elevated Windows cmd gets things booting to GRUB, and from there you can select Ubuntu or Windows. I'm not 100% sure what that does, but it may just change the "Windows Boot Manager" path in the EFI config.
As a workaround, a "bcdedit /set {bootmgr} path \EFI\ubuntu\ grubx64. efi" in an elevated Windows cmd gets things booting to GRUB, and from there you can select Ubuntu or Windows. I'm not 100% sure what that does, but it may just change the "Windows Boot Manager" path in the EFI config.