That sucks.
Line the piston you have up with your port map. Place the piston at the bottom of blowdown - just at the point where the transfers are open.
Now look at where the piston lines up at the boost port. Is the piston close to those little bridges or? I'm thinking the piston rocked slightly inward a bit and overheated on the intake side. Does the piston show wear at the skirt bottom intake side? Were the piston edges chamfered?
Is it possible to find cast piston for RM500 from ProX/Art/Wossener? What does
CW Racing sell? Wiseco forged pistons need more clearance. I'm thinking if you can use a cast piston with tighter clearance this type of issue won't happen.
Or I guess you could have LA Sleeve leave a few more mm of bridge material.