What exact procedure of maintenance do you do? Did you just use compressed air to clean them up?
How about the temp and fan speed?
If it worked before, I believe this undetectable ASIC chips has poor contact with the heatsink.
If you can disassemble the unit again, I recommend cleaning it up, removing the old thermal paste, and replacing it.
Also, test them one by one because sometimes other hashboard affects the other hashboard to send false signals.
I haven't touched the miner. As I stated in my OP, I shut down power to the miner as I was conducting non-related maintenance at the site.
The miner was literally working perfectly, turned off for 10 minutes or so (while I changed a breaker in the main panel), then when it was turned back on it had 3 dead hashboards. If I had been working on the miner itself, it would make more sense!
I have since been down to the site and changed control board and PSU, but neither fixed the issue. It just literally went from working perfectly to 3 dead boards after a power recycle. Very strange!