Can Plennyshakes Go Bad during the Summer?

Hi, due to some Unfortunate Decisions (laziness), I’ve kept my sealed Plennyshake Chocolate bags in my room during the last two weeks’ 29°C weather. Now I just opened one and it smells a bit weird (slightly sour?) but that might just be me. All the bags still had six to nine months before official expiry.

Can sealed Plennyshake go bad in summer heat?

My sealed bags (chocolate) that have been around for several weeks and get up to around 29 each day are fine.