I think having a bag of spare Matias keyswitches around and occasionally replacing them when they die (and don’t respond even after isopropanol) is just a fact of life. I’ve just replaced my second one (first was space, second is e, so approximately the most used keys).
Matias switches have a great keyfeel but they clearly have some reliability issues.
Good news is it’s fairly easy process if you have a soldering iron and a solder sucker. Seven big screws to remove the case, seven small screws to separate the switch board from the rest of the circuitry, then desolder the two contacts of the bad switch, pull it off, pop the new one in, and resolder. Advice from Jesse: “The big issue is really to make sure you don’t rip out the electrical contacts when pulling out the switch after desoldering. Just making sure there’s a clear gap between the contact and the through hole for each of the switch’s two contacts before you yank it out ought to be enough for that”. Advice from me: when you come to put the case back on and find you only have seven screws for eight holes, don’t panic, you didn’t lose a screw, there never was one in the central hole.
(Or if you really don’t want to do it yourself you could email support@keyboard.io and ask if they’d be willing to do it for a fee)