MacBook Air 2018 keyboard backlight problem

I've got a 2018 MacBook Air with a keyboard backlight problem. While running macOS, there is no keyboard backlight, the F5-F6 keys won't control it, there is no backlight option in the Keyboard System Preferences pane, the Control Center Keyboard Brightness panel exists, but when clicked the slider is at the leftmost (off) position, sliding it does nothing, and when released it immediately jumps back to the leftmost position.

Things I have tried so far:

Reinstall macOS

Wipe and install macOS 10.14

Wipe and install macOS 10.15

Wipe and install macOS 11

Drop into DFU mode, use Configurator 2 to reinstall T2 firmware, then install macOS 10.14, 10.15, and 11.

Ok, backlight hardware issue, right?

BUT NO!

When FileVault is turned on, after shutting down and powering on, the keyboard backlight WORKS!

Ok, enter password, hit return.

Backlight fades out, and does not return until next shutdown/restart, and then is active only until FV is unlocked.

It passes Mac Resource Inspector and Full System Diagnostic (EFI) Apple Service Tests.

So, not hardware. Anybody seen this before? Have a fix?

It's out of warranty, no AppleCare, so I don't want to start swapping parts.