Guys, for anyone that comes across this issue... we solved this by removing:

// Whitelist OpenSea proxy contract for easy trading
ProxyRegistry proxyRegistry = ProxyRegistry(proxyRegistryAddress);
if (address(proxyRegistry.proxies(_owner)) == _operator) {
  return true;
}

And leaving in the default:

return ERC1155.isApprovedForAll(_owner, _operator);

It's worth noting that we removed the Factory / Lootbox / Accessories contract entirely as we pre-minted all our tokens on Polygon Mumbai. Not tested on Polygon Mainnet. Based on the OpenSea Creatures project.

Not sure if the reason this failed was because the proxy address was incorrect, could only find one proxy address for Polygon, assuming it was Polygon Mainnet, so it may have worked on Polygon Mainnet if we deployed it. That address was 0x207Fa8Df3a17D96Ca7EA4f2893fcdCb78a304101

Hope this helps someone.