The viewer used by comic.pixiv is incredibly bothersome, since unlike some viewers, you can't just use the usual Web Developer tab solutions, like the Network console on Firefox for example.
When I was uploading Risubokkuri, I ended up being faced with the same issue (since the alternative, that being the original Comic Gum version, had scrambled images for the source and the viewer descrambling said images). I don't exactly recall what I did to obtain those images, since that was months ago, but looking at a few chat posts elsewhere, I believe I was shown the way via viewing the browser's cache, and then just resaving those images already in its cache. So, if there's any method to obtaining the images from the viewer I'd recommend, it'd be through a cache viewer.
Now, with that out of the way, I'd also like to point out that the comic.pixiv version of this story isn't actually part of the continuation of the story thus far, but a continuity reset, since this is actually the serialized version! As the page states, and is evident in the chapter title, it's part of Comic Pool's line-up, akin to Wotakoi (which also has a comic.pixiv version, which has differences from the published manga release [and said revamped manga chapters were uploaded to pixiv, and subsequently here]); and you can tell it's a continuity reset because Futaba's got her red hairband, rather than the purple one she had since Chapter 34. If we figure out a practical means of uploading from comic.pixiv, it may be worthwhile to make a new pool.
You can get the image URLs from the developer console or similar request logs and then use wget, curl or any other downloader that can set the referrer URL to the story URL to download them.
(If you’re on Windows, you might need to use double quotes instead of single quotes around the image URL.)
If you redownload these upscaled and horribly artifact-ridden images, upload them to a host that doesn’t alter images (like catbox.moe) and request replacements on the forum.
Edit: I guess BlastingNaba figured it out already, but I’m leaving it here for anyone else interested.
