If Windows update overwrites the Grub bootloader. Simple way to restore is to copy the debian bootloader (grubx64.efi) over the windows bootloader. We do the following steps:

1. Save the bootloader installed by Windows: copy E:\EFI\Microsoft\Boot\bootmgfw.efi E:\EFI\Microsoft\Boot\bootmgfw.efi.original
2. Copy the debian grub bootloader over the bootloader installed by Microsoft (bootmgfw.efi). copy E:\EFI\debian\grubx64.efi E:\EFI\Microsoft\Boot\bootmgfw.efi
3. Add an entry in Grub boot menu to start the orginal windows loader.

Open CMD prompt as “Administrator” and mount the EFT partition as E: drive with:

mountvol E: /S

E:\EFI>dir
 Volume in drive E has no label.
 Volume Serial Number is 502B-5BC6

 Directory of E:\EFI

06-12-2016  20:05    <DIR>          .
06-12-2016  20:05    <DIR>          ..
06-12-2016  20:05    <DIR>          Microsoft
13-02-2021  19:01    <DIR>          Boot
27-11-2021  20:18    <DIR>          HP
13-02-2021  08:35    <DIR>          debian
               0 File(s)              0 bytes
               6 Dir(s)     160.800.768 bytes free

copy the debian grub bootloader over the bootloader installed by Microsoft (bootmgfw.efi).

copy E:\EFI\debian\grubx64.efi E:\EFI\Microsoft\Boot\bootmgfw.efi

Add the following lines to: /etc/grub.d/40_custom

menuentry 'WIN 10 (on /dev/sda1)' --class windows --class os $menuentry_id_option 'Win10-efi-502B-5BC6' {
        insmod part_gpt
        insmod fat
        set root='hd0,gpt1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt1 --hint-efi=hd0,gpt1 --hint-baremetal=ahci0,gpt1  502B-5BC6
        else
          search --no-floppy --fs-uuid --set=root 502B-5BC6
        fi
        chainloader /EFI/Microsoft/Boot/bootmgfw.efi.original.20221209
}

C:\WINDOWS\system32>bcdedit

Windows Boot Manager
--------------------
identifier              {bootmgr}
device                  partition=\Device\HarddiskVolume1
path                    \EFI\Microsoft\Boot\bootmgfw.efi
description             Windows Boot Manager
locale                  nl-NL
inherit                 {globalsettings}
default                 {current}
resumeobject            {6beefd36-680a-11eb-ac07-a7444769703e}
displayorder            {current}
toolsdisplayorder       {memdiag}
timeout                 0

Windows Boot Loader
-------------------
identifier              {current}
device                  partition=C:
path                    \WINDOWS\system32\winload.efi
description             Windows 10
locale                  nl-NL
inherit                 {bootloadersettings}
recoverysequence        {5c5834f8-aff6-11e9-ac26-ce270aa00281}
displaymessageoverride  Recovery
recoveryenabled         Yes
isolatedcontext         Yes
allowedinmemorysettings 0x15000075
osdevice                partition=C:
systemroot              \WINDOWS
resumeobject            {6beefd36-680a-11eb-ac07-a7444769703e}
nx                      OptIn
bootmenupolicy          Standard

bcdedit /set {bootmgr} path \EFI\debian\grubx64.efi
bcdedit /set {bootmgr} path \EFI\Microsoft\Boot\bootmgfw.efi

bcdedit /enum firmware
bcdedit /timeout 10
bcdedit /copy {current} /d "Linux"

Windows Boot Manager
--------------------
identifier              {6beefd39-680a-11eb-ac07-a7444769703e}
device                  partition=\Device\HarddiskVolume1
path                    \EFI\Microsoft\Boot\bootmgfw.efi
description             Linux
locale                  nl-NL
inherit                 {globalsettings}
default                 {current}
resumeobject            {6beefd36-680a-11eb-ac07-a7444769703e}
displayorder            {current}
toolsdisplayorder       {memdiag}
timeout                 10