Question: Are the rings still free in the grooves?
Next,- notice the scuff marks I can sort of see on the piston skirts. (scoring)
In there is probably a lack of lubrication on the bottom and detonation on the top.
If the rings are not free to move, this happens if the piston is slowly compressed by the detonation shock waves .
But if it is a cast piston, they just break. (as it did)

The piston looks like the rings butted, due to a lean condition, then broke the lands. The melting happened after that. Additional oil promotes the melting of the piston, which is evident. This engine was driven fairly hard after the rings butted. Loss of power and excessive oil smoke out the exhaust, were tell tail signs. It was driven hard with a problem. At 1500 miles, the problem isn't yours.

I didn’t read all the responses, but was there too much play between the rings and the lands? Too much play, and the ring chatters in the groove, breaking the lands.

At least that’s what I recall.