No keyboard backlight until password entered
When FileVault is enabled it encrypts everything on the SSD and doesn't allow even the computer to have access to that data until you enter your password. Including the software that loads during startup which would normally activate the backlighting on your keyboard and the touch-bar.
After you enter your password on startup the decryption process begins and all of those necessary pieces of software start to load. Making your keyboard backlight turn on and your touch-bar activate.
Here are your 2 choices:
- Accept it and remain using FileVault (my recommendation)
- Turn off FileVault (which I really do not recommend)
Assuming you are FileVaulted, fold your screen down just short of crushing your fingers. The light is sufficient to illuminate the kbd enough to be able to enter your pwd.