I love my Logitech MX Keys keyboard, but it has its flaws.
My spacebar came loose within few months of use, and it seems that carrying it in my backpack might have put excessive force on the mechanism, breaking off the clips that hold the key into place.

I found this fix on YouTube, showing the mechanism: https://www.youtube.com/watch?v=r1ygLsL4MM8&t=177s
However when the plastic keycaps are damaged, you will have likely noticed some crunchy sounds underneath your keys as those tiny clips had broken off.

The spacebar is additionally held into place using two metal brackets, which prevented it from falling off, but without holding on to the scissor mechanisms, the key doesnt hold in place.

To fix my spacebar, I glued replacement clips to the keycap which then clip to the scissor mechanisms.

• Remove keycap
• Shave flat the inner surface of the spacebar where the white mechanism clips broke off.
• Print two replacement clip pads, then clip them onto the white scissors
• Clip in the bottom metal bracket next. Then add a small drop of superglue (Cyanoacrylate) onto the flat pads
• Push the spacebar back in, while also clipping the top spacebar clips to the top metal bracket. Immediately keep applying pressure at either sides and align the spacebar to center it. Use popsickle sticks and a woodworking clamp to secure the spacebar for at least 30 minutes until it hardens.

My spacebar is not back to the original feel, but much better than barely hanging on. The alignment is crucial or the spacebar will be crooked and not press correctly. However superglue comes off the keycaps easily so it can be realigned or repaired multiple times.
The spacebar can also be tuned by taking out the center metal bracket and bending it to a slight "U" shape, then clip it back in. This gives the spacebar strength to push down on the key membrane in the center (which is arguably a terrible mechanism). Just dont bend it too much, or the spacebar will constantly press down when clipped in.

This is still a WIP as the tolerances are small and the design could be further improved. It might be adapted to fix other keys as well in the future.