I'm not currently on the MD2 redesign, but I don't think that should be relevant to this bug. Take a look at this:
https://i.stack.imgur.com/2kbGG.gif
This image was generated for the sake of this page:
https://math.stackexchange.com/a/639079
My guess is that imgur doesn't automatically convert gifs to mp4 videos on its stack subdomain, but Sync is using a regex or similar that fails to account for that. Alternatively, it's possible that this gif to mp4 conversion generally doesn't work for still images. Either way, Sync fails to load the image.
Scenario (steps to reproduce)
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Click on the above .gif link
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Watch as Sync fails to load the image
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Note now it still works on