I would not use triple thick for that… instead try Minwax rattle can “fast drying” clear poly. I used on Space shuttle a couple years ago and it came out decent.

https://pinside.com/pinball/forum/topic/the-spraymax-2k-auto-clear9-in-a-can-club

https://pinside.com/pinball/forum/topic/vids-guide-to-ultimate-playfield-restoration/page/16#post-1667058

That is a reaction with something on the insert. Sand it down and clean it to make sure there isn’t any contaminants before re clearing it.

Yes I don't think it was really clean before spraying and light coats let dry sand ,clean coat again until it comes out good then wet sand with 1500 grit then buff

Non-yellowing water-based polyurathane or varathane are typically the family of products folks seem to use if automotive clear coat isn't an option.

Triple thick isn't really the right product to use in this situation.

On that era playfield it will be crucial that the inserts are stripped bare of any factory coating prior to clearing.
I probably would not use triple thick either.
Not really well versed on the spray can clears but whatever the community suggest is probably a reliable source in that regard.
The System 11 playfields are notorious for this reaction and even some prior but within that timeframe.
The coating used on the initial screening just did not bond well with the plastic inserts. As a result pretty much anything you spray on them acts as a stripper and loosens the factory finish from the insert.
If there is no factory finish then the problem is solved prior to recoating.

Sand the inserts then use SprayMax straight urethane two part. You can only buy this stuff online and be sure to use a mask.