Sunlight affecting resin

I’ve fairly new to resin and thought i’d been doing ok…until the sun FINALLY decided to show its face!!! My resin is hard, not sticky, cured without any issues in the timeframe mentioned in the instructions, measured to the Nth degree, was mixed after warming through, containers not scraped when pouring. Everything that usually causes issues I’ve avoided. So…here’s the question. Will full on direct sunlight soften it up slightly and make it slightly pliable? No full on bendy…just ‘moveable’. I only noticed this after some pieces that had been stored in the house got moved into my conservatory on a really hot day for an hour or so. It was about 25 degrees C outside, but was more like 30 degrees C in the conservatory and the pieces were in full sunlight. Once i brought them back inside that were back to being rock solid within about 5-10 minutes. Is it me…is it the sun? I honestly can’t think what i’m doing wrong.