So I opened up Osu! this morning and found my Osu! avatar to be empty. This did not surprise me at first as I had recently tried to change my avatar but failed due to filesize being too big. However, when I checked my profile later, I found that the avatar that I tried to replace is still up on the profile page. Following is what it looks like from my side.
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Avatar issues [invalid]
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It'll take a while for it to change in-game as far as I know.
Indeed. It can take several days unfortunately 
There was a similar issue to this fixed just recently. If you're still experiencing this, then it's a result of ISP caching, and there's nothing you can do but wait.