Wow that’s unfortunate, looks like the new clear drops attacked the existing. Did you allow the last coat of clear to fully cure before applying the drops? I’ve never cleared a playfield, but had something similar happen with regular paint when applying more too soon. As I understand it, you have a short window of time to add more, otherwise you have to wait days for full cure. Plus maybe applying a concentrated amount like that contained too much solvent for a small spot. Sorry this happened

Maybe the clear isn’t able to adhere to the recessed area around the insert because it needs to somehow be abraded inside the ring/crack. When you block sand it’s making the surface smooth, but maybe the clear in the ring below the surface is still smooth and nothing sticks to it. Just a thought....