What you’re describing is kinda weird. The code only gets corrupted if the chip actually fries, and at that point you gotta swap the whole PLC. What’s probably happening is those PLCs are kinda old, so they’re losing their config or parameters, which are stored in a different type of memory than the logic. I mean, sure, in super rare cases the flash memory might go bad, but usually you can’t just reflash the logic; when it dies, it’s dead for good. It’s pretty normal for regular folks to mix up re-flashing with re-configuring.