We are not quite certain what is causing this issue - we think it is down to the amount of pressure on the mylar as it basically just sits against those fingers.
You are the second person to report that only the BREAK key is working whereas another person has about half the keys working without changing anything.
What we think is required is a small piece of material (such as a thin piece of foam as used on the Commodore SX-64) or maybe even a piece of paper folded a couple of times, between the mylar and the small plastic bar which holds the mylar down.
We have suggested this to the affected users, but not yet received any feedback as to whether this helps and if so how thick it needs to be. It might just be a matter of us provided an extra thin sheet of plastic to make the mylar that little bit thicker overall.
It may be worth trying a couple of sheets of paper behind the whole of the mylar - Jerzy Dudek says some 800xl mylars have the same issue (where the keys are slightly worn).
Unfortunately, the factory (based in Scotland) is struggling with lockdown and not operational at the moment, otherwise they could borrow Adam's Atari 800 again and try a range of materials to work out what is required.