It could be that the heating/cooling of the VRM is performing unintended metallurgy on the components of the VRM itself. That may explain why it quickly burned up after 2 weeks of no usage.

This of course could be the fault of a VRM manufactured with poor quality materials, or it could be the fault of a poorly cooled VRM. Or both.

Roman, did you happen to notice which VRM failed? Was it the rightmost VRM under heatsink 1?