I’ve seen this issue twice – first on a client’s machine that got kicked out of the skies by a 1803 update and later, in 2020, on my expedition laptop that I carelessly launched the Kali usb drive executable on.
Pure curiousity that delayed the launch of the Win10 for 30 seconds offering me the choice to either boot into Windows or select Kali installation. Given that I work in Windows and occasionally run Kali for training and recreational purposes, this was a sacrifice I was not going to tolerate.
After half an hour of rigorous search of the place where this Debian installer could root itself to affect the Windows boot selector, I’ve found the following menu:
Control Panel > System > System Protection on the left > Advanced tab > Startup and recovery > Settings > System startup.
There you have to change the Default operating system to Windows 10, disable the Time to display list of operating systems and click “OK”.
Voila, you’re good to go.