Broke "sticker" in the butterfly keyboard while cleaning. What is it?

The J key stopped working on my MacBook pro as it did many times in the past. I initially set the MacBook on its side and sprayed the keyboard with compressed air. No dice.

I took off the keycap by inserting a bit of plastic at the top and being careful not to break it. I once again set the MacBook on its side and started spraying air.

I probably got a bit too eager with it, because this little bit of what looks like a black sticker came flying off.

I picked up the sticker and haphazardly put it back where it was. The key still skips a few beats, but it works better than before. However I'd like to understand whether I should be worried about this broken bit.

The J key with the (black) broken sticker at the top

Comparison with a "healthy" G key (compare the left and right corners of the sticker)

What is this thing? Is it critical to the function of the keyboard?