Not sure if this is isolated to myself, but when I tried to see if https://pawoo.net/@junyurijp/5317580 had been uploaded by using the bookmarklet, but nothing similar came up. I then tried using the bookmarklet to upload it from pixiv to see if that would trigger it and it still did not. The post has been uploaded before at post #2446059
Similar testing against post #2448150 had no tripping of it. But tests against post #2689672, and post #2703424 do trip the similarity trigger.
I'm not sure if it's a problem just for the artist in question vashaps2 or what is going on.