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

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