It seems the flash is damaged on your switches - Where did you buy them? You might have gotten a bad batch (the flash chip on these is pushing 10 years old, although it's rare for this many to fail at once). You could have also had a power surge or lightning strike nearby that induced enough voltage to damage the flash IC. This definitely isn't caused by the flash guide, which is why nobody else has had the issue. There's no way for a bootloader and software designed for this flash chip and CPU to slowly damage it somehow
Where did you buy them? If you bought them on ebay, which seller? I know one seller was purposefully trying to break the flash with a drill - it's possible the damage wasn't bad enough to notice at first but is now finally failing due to the damage. If you open them up look for the flash chip and see if it has any marks like this - https://i.imgur.com/6VCxXep.jpg
Where did you buy them? If you bought them on ebay, which seller? I know one seller was purposefully trying to break the flash with a drill - it's possible the damage wasn't bad enough to notice at first but is now finally failing due to the damage. If you open them up look for the flash chip and see if it has any marks like this - https://i.imgur.com/6VCxXep.jpg