Hi, I have the same problem.

With bitcore-mnemonic, the library generate the mnemonic for an extended private key.

According to
https://www.ethnews.com/jaxx-to-abandon-ethereum-hierarchical-deterministic-structure-and-embrace-more-erc20-tokens
I don't want to use an HDWallet for Ethereum.

So, is there a way to generate the mnemonic from a private key (not extended) ?