Yeah it's not Pinterest, it's Google image search. It's been horrible for many years now, but really got bad when they stopped letting you go straight to the image and instead forcing you to the page the image is on.
The effect of this is it's in fact muuuuch easier to get at the original image in image search. Just right click the image and hit "view image" or "view image in new tab" and, that's the original.
What's annoying though is that Google doesn't show you the image sizes anymore. That's also relatively recent.
>A Pinterest spokesperson tells Input that the problem doesn’t lie with the company, but with search engines. “As a search engine, Google controls how results appear based on engagement and how useful searchers find the content.”
Google: "We allow shitty things because money."
Pinterest: "also money."
EDIT: strange, i tried the same process again on the same image and this time it worked! i guess it really is a cache-related issue