Heat Separation of Resin Layers

Had an unfortunate resin experience lately, trying to get to a solution: several hundred botanical resin necklaces in my jewelry display cases were momentarily in the sun (it’s California summer here so even a few minutes in 100+ degrees Fahrenheit can skyrocket the internal temp like a convection oven), I opened my cases immediately and blocked the sun but experienced bubbling and subsequent cracking within the necklaces between the layers of resin. Each piece has 3-4 layers.

I’m trying to figure out if this was caused by a chemical heat reaction of one of the following and how to prevent it in the future: either there were bubbles hiding between the tumbled gemstones or botanical elements I used, or the mod podge reacted to the heat, or the resin itself started to react. I can’t figure out how to post images to show you what happened, but I used resin obsessions resin, leaves were fully dried and double sealed with mod podge. Several of these necklaces were over a year old with no issues until this point, so I’m at a loss I’d how to fix this to prevent future loss of merch! Thanks in advance for all your time, help, and wisdom!