The share slot has a shape: roughly 1200x630, a 1.91:1 rectangle. Everything that isn’t that shape gets negotiated by the platform, and the platform always wins.
The usual suspects are assets that already existed for another job. Sprite points its og:image at a 477x180 logo, so the large card it declares gets a tiny wordmark upscaled into fuzz. Spindrift ships a page banner, wide and thin with the subject off to one side, so the centre-crop cuts the subject out. SoundCloud ships an asset literally named unfurl-square, album-art instincts in a widescreen world. Twitch points its tags at a logo and declares the small summary card on top of it, so a video platform unfurls as a thumbnail with nothing to watch.
None of this shows up in a tag validator, because the tags are valid. The failure is aesthetic, it happens per platform, and the person who shipped the tag usually only ever checked one.