Single key without backlight.

Hi, thanks for your time.

No liquids spilled over the keyboard, I'm super cautious with this.

I noticed the problem after upgrading to BigSur, but not sure if it was lighting up before.

Brought the machine to apple and they performed a Firmware an OS reinstall without success.

This is an A1989.

On December 2020 It had the keyboard replacement program, mother board got cleaned. It had replaced everything but the motherboard and the screen.

So the keyboard and the back light is only 10month old.

I live in a very humid area (north Colombia, south america), the apple technician told me that in some cases accumulated dust under the keyboard might catch enough moisture from air in order to damage circuits, but since its only a 10month old machine it is very rare. He see those dust problems in 2 year old machines and older...

any way apple technician thinks it could be solved in the next update in case is a software problem...

Is there any chance you have an illustration of how does the light pipe work?

@Giandaniel G - Very odd! If a backlight LED had died then you would have seen the neighboring keys also effected.

I think you'll need to get your system repaired again under the keyboard program. You did say you had a few keys acting up again right??? ;-}

I'm betting the guide got dislodged or something like a piece of paper slid in-between the backlight pathway.

Sorry to say Big Sur would't have caused this.

Take the time to review this MacBook Pro 13" Touch Bar 2018 キーボードの分解

The keyboard is made up of a sandwich the backside is the middle picture in Step 5 which is the key switch plate and backlight. Its made up of a bunch of LED's which are scattered around the keyboard throwing light out which is sent into the white plastic parts (pipes) to the keycap.