Long black screen before shutdown on Windows 10

My ThinkPad P50 has been having some difficulty shutting down lately. Any time I shut down or restart, after the usual Windows shutdown process, the following happens:

  • The screen turns off completely (backlight included);
  • The Num Lock and Caps Lock LEDs, as well as the power LED turn off;
  • The audio mute LED remains on.

This can last just one or two seconds on the short end, but I’ve had it last over five minutes, at which point I killed it using the power button.

Additional info:

  • Windows is installed on an M.2 drive;
  • There is a SATA drive with a Debian installation on it (I believe the start of my issues more or less coincides with this drive’s installation, but it’s been to long to remember if it matches exactly);
  • The Windows drive is BitLocker encrypted;
  • Secure boot is off;
  • Windows Boot Manager has been exchanged for GRUB;
  • Linux experiences no such issues.

As nowadays I often switch between the two operating systems, this can get seriously annoying. Thank you in advance for the assistance.