PCB plated through hole short-circuit
On such wide question I can only give research questions.


Validate if the board layout is correct.


Check if the continuous voltage present provides enough clearance and creepage.
Check for any surges or spikes, the re-validate the the board design.
Check for current rating and track/via within normal conditions.
Check for current ratings in abnormal conditions. (add fuses is necessary)

Validate the Assembly is correct.


Are there lots of tiny solder balls after soldering? Improve reflow/solder process.
Is there manual soldering used? Too hot, too much, leftover flux of bad kind?

I can't see if there used to be a component where the board has vanished, what if this component is abnormal or is used incorrectly?
Is the operating environment clean (pollution degree) and how is moisture?