I never store 12 inch in boxes, all of them are in my display cabinet. My Hakaider also suffered coat melting at the area of belt. I am using electrical dehumidifier and relative humidity is ~50% all along. It seemed that pressure and humidity also affect 12inch.
After this, I loosened the belts of all 12inch and avoid melting at the contact points.
I also heard from SupermXX that one big big suffered from coat melting after prolonged contact between 2 riders.
So I also keep all 12inch from contacting each other.
At the present moment, some of the ways being heard:
1.low humidity: best to open up and display.
2.NO contact point with high pressure.
3.avoid figures touching each other
4. avoid overheating by sunlight exposure.