It is really hard to tell,I don't think it's so much that the food goes bad,it could just be a difference in the production,or once the can is opened and left,it can go bad just like anything else.
I keep my canned cat-food in roomtemps,the same as it would be in the store.
My cats would not eat food coming from the fridge,unless I warm it up in the micro.
As a rule,if I open a can and the consistence of the food is very different from the way it usually is,I throw it away.
Wellness is a very good food and you're lucky your cat eats it,mine don't,they have good quality dry food,but canned they will only eat the dreaded Fancy Feast
However they only split a small can between them twice a day.
I am sorry about your kittys illness,I hope he recovered well :love: