I know it looks like it could be Shopify which has done something. But from what I saw the progress Shopify has been making in the last many months the sites have just faster and faster.
I would rather bet on Google is tweaking their algorithms which results int them changing your score. I saw a similar thing happen at my store. I’ve managed to fix most of them but I’m still struggling to figure out why I’m getting so high CLS on desktop since neither PSI nor Lighthouse reports any issues.