I don’t actually ever invoke any bootloader directly, but instead use an EFI app which in turn loads the bootloaders.

Windows’ Anniversary Update works by adding an additional, hidden EFI bootloader, and loading that for the upgrade.

If you instead load the normal Windows EFI bootloader while the upgrade has half-way finished, then it fucks stuff up.

I’ve been through it multiple times, last time, Windows even destroyed all its own EFI entries.