Related rant: Also I partly (A BIG PART)blame Microsoft for this kind of bs. That being the way they've designed windows OS's or whatever to seize back control of the boot process from the way the pc owner has configured it. Someone goes to update Win8-10 whichever, ... nope afterwards can't boot their other XYZ-OS's anymore, not without having to repair it's bootloader etc blahblahblah. They of course claim this is for security or whatever other nonsense but fact is they do it to screw with people trying to multiboot with open source alternatives, mostly gnu/Linux users, though also BSD'ers or whoever else too.
There are ways to set your preferred boot manager in a windows admin command prompt. I've done it, think it was Win8 and 8.1 at that time, don't remember the process and would have to google myself. Originally learned how to do it by reading posts from the guy behind rEFInd, (Rod Smith.) Though there are plenty of how-to's and docs on how to do it online. Worked for a long time, Windows left grub2 alone thereafter ...
Eventually I went with a universal solution for every/all Window$ related computer problems, stopped using anything Windows and went exclusively gnu/Linux several years ago. I haven't had a single M$/Windows related computer problem since. Also just 4 record, I've actually liked pretty much every M$ OS I've ever used, even friggin Vista. Though AFTER having to do a chit-ton of changes, tweaks and optimizations, perform surgery to remove spyware etc etc to the damn OS's.
More stuff ... at one time used and liked a freeware application called EasyBCD, let's someone use the Windows boot manager but boot other OS's. Not sure as to its status, been a longgggg time. So not sure if they support secureboot + UEFI + GPT now or not. Though during the time I was using it, it worked well and liked it. Let me set grub as the default OS to boot with a 0sec timeout. So basically, as soon as the system was booted I'd wind up at the grub boot-menu screen and then if wanted had it set so I had a couple secs to choose to boot Win7-8 or whichever from there.
Finally, there's the BIG DADDY of gnu/Linux repair, chroot from a live session, a person can do a dizzying amount of things from a live chroot. Repair, replace, reinstall-reconfig pkgs etc etc etc. That would of course work for this, that being allow you to chroot the borked install and use efibootmgr to set the boot manager you want or reinstall grub onto the system if necessary.
One live chroot how-to:
https://forums.bunsenlabs.org/viewtopic.php?id=1192