Tank pad problems

[Renzakuken]

Hello all

I purchased the following tank pad a year ago and happily had it suckered onto my last bike:

https://www.infinitymotorcycles.com/product/spada-suction-tank-bag-3-litre-black?gclid=CjwKCAjw47bLBRBkEiwABh-PkcNJQpgREc4-HdJoiaouRTWcMoBBlLnd9vkqHnbBv94IrTP-rXxV_xoCMoEQAvD_BwE

I noted however that in the warmer weeks i'd literally have to peel it off, there was a thick sluggish liquid on the underside. I thought it was the lining starting to melt in the heat so just wiped it down and place it back on the tank.
I came to sell the bike, peeled it off again and saw that an area around the fuel cap had started to peel, specifically the glossy top-layer of the tank.

two questions:

1) Am i right in thinking the fuel cap wasn't entirely sealed and so vapors from the tank were trapped between the cap and the bag
2) How can I guarantee this won't happen again
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CBR 125 2006 STOLEN
GSX600F 2003 SOLD
GSX650F 2009 CURRENT