How to properly implement the ContractURI for full on-chain data?

I have this ERC721 contract (removed all unnecessary stuff), in which I also implement TokenURI using base64 encoded JSON and SVG. The on-chain TokenURI works and OpenSea renders it properly. But the ContractURI, when set to the example (without comments, so valid JSON) does not change the store page at all.

I've followed the steps on the OpenSea page about contract metadata.

Am I doing something wrong, or have I missed something?

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If this sub isn't the proper sub for it, let me know or delete the post. I've also asked on OpenSea and Ethdev, to no avail yet.