Hey @Vinh_Trinh and @oxuw4. I came across the same issue and just found the root cause: Unidentified contract: OpenSea is unable to "understand" ERC-1155 - #2 by itinance
Long story short: add a public variable called name into your contract (and probably a property symbol as well) and your contract will be shown properly on OpenSea. This here is my working example, based on OpenZeppelin ERC1155: https://testnets.opensea.io/assets/0xfdBd4652390618aca3f60c9B78c4eE9AAFD74914/1