So the fix is really simple.

The idea is to add a shim between the stripe containing the keys buttons and the plastic pink supports on the bottom.

For some reason there is a 7mm gap between the two parts, allowing the keys in the middle to easily bend the circuit stripe, absorbing the finger pressure needed to activate the buttons.

I happened to a have a 5mm thick wood scrape, leaving 2mm of slack. I think that a whole 7mm shim would make the key too stiff.

The scrape shim I found was only 25cm long and the circuit stripe is about 30cm. So maybe a longer shim would be better.

There are some standoffs at the side of the pink plastic supports. So if your shim is larger than 17.5 cm be prepared to cut some hole to the side of the shim to make place for this standoffs.